Add support NUnit XML format

[bryanbcook]

Addresses #4

• [x] NUnit V2 Support
• [x] NUnit V3 Support

The formats for test-suites and test-case between V2 and V3 are similar, but have some structural and summary differences.

V2:

• top-level node is test-results. The attribute total represent the number of executed, where as not-run were not executed. There is no total for passed, it must be inferred from the total by subtracting errors+failures+inconclusives.
• test-suites are nested within results nodes, with each part of the namespace represented as a different node. As a result, the depth of the structure is not fixed.
• Names for test-suites are partial, whereas the name of the test-case is the fully-qualified namespace + class name + method.
• The test-case nodes are held primarily in test-suites that have a TestFixture type, with the exception of ParameterizedTest.
• Properties/Categories can be set at the fixture of test-case level.
• No id attributes on nodes.

<test-results total="n" errors="n" failures="n" not-run="n" inconclusive="n" ignored="n" skipped="n" invalid="n">
  <test-suite type="Assembly" name="/full/path/to/assembly.dll">
    <results>
      <test-suite type="Namespace" name"="NamespaceName" >
        <results>
          <test-suite type="TextFixture" name="ClassName">
             <categories>
              <category name="..." value="..." />
             </categories>
             <properties>
               <property name="..." value=".." />
             </properties>        
             <results>
               <test-case name="NamespaceName.ClassName.MethodName"
                          executed="{true|false}" 
                          result="{Success|Failure|Ignored|Error|Inconclusive|NotRunnable}" 
                          success="{true|false}">
                  <categories>
                    <category name="..." value="..." />
                  </categories>
                  <properties>
                    <property name="..." value=".." />
                  </properties>
                  <!-- either failure or reason (NotRunnable) -->
                  <failure>
                    <message><![CDATA[ ... ]]></message>
                    <stacktrace><![CDATA[ ... ]]></message>
                  </failure>
                  <reason>
                    <message><![CDATA[ ... ]></message>
                  </reason>
               </test-case>
             </results>
          </test-suite>
        </results>
      </test-suite>
    </results>
  </test-suite>
</test-results>

V3:

• top level node is test-run. Additional attributes for testcasecount (previously total + not-run), passed (previously total - errors+failures+inconclusive).
• test-suite nodes are not deeply nested, though
• test-suite now has fullname
• test-case no longer has executed or success attributes. The Label attribute contains Error, Cancelled or Invalid if there's a Failed state.
• test-case result can be Passed (instead of Success), Failed (instead of Failure), Inconclusive or Skipped.
• V2 previously included test-cases for invalid test-fixtures, which were marked as executed="False". V3 does not include these test-case nodes, but the test-suite counts them in the testcasecount attribute, so it's possible to have an empty test-suite that indicates it has test-cases but it does not.
• Support for <properties> but not <categories>. In addition to fixture and test-case level, properties are also supported at the test-suite Type="Assembly" level.
• id attributes are present.

<test-run id="n" result="{Passed|Failed}" testcasecount="n" total="n" passed="n" failed="n" inconclusive="n" skipped="n" >
  <test-suite id="n" type="Assembly" fullname="/path/to/assembly.dll">
    <properties>
      <property name="..." value="..." />
    <properties>
    <test-suite id="n" type="TestFixture" fullname="NamespaceName.ClassName"
                result="{Passed|Failed|Inconclusive|Skipped}">
      <properties>
        <property name="..." value="..." />
      <properties>
      <test-case id="n" name="MethodName" fullname="NamespaceName.ClassName.MethodName"
                 result="{Passed|Failed|Skipped}"
                 label="{Error|Cancelled|Invalid}">
        <!-- either failure or reason (Invalid) -->
        <failure>
          <message><![CDATA[ ... ]]></message>
           <stacktrace><![CDATA[ ... ]]></message>
        </failure>
        <reason>
          <message><![CDATA[ ... ]></message>
        </reason>
      </test-case>
    </test-suite>
  <test-suite>
</test-run>

This implementation automatically detects whether you are using nunit v2 or nunit v3 format.