Add support for embedded struct fields

[awoodbeck]

This change is meant to address #58 and recurse into embedded structs, pointers to structs, and named structs in order to build the fields map. I also addressed a few linter complaints.

[icholy]

The problem here is that the marshaling/unmarshaling is not symmetric. Here's what the embedded struct (from your tests) gets marshalled to:

<?xml version="1.0"?>
<value>
  <struct>
    <member>
      <name>Name</name>
      <value>
        <string>Blah</string>
      </value>
    </member>
    <member>
      <name>Status</name>
      <value>
        <struct>
          <member>
            <name>Code</name>
            <value>
              <int>1</int>
            </value>
          </member>
          <member>
            <name>Msg</name>
            <value>
              <string>Test</string>
            </value>
          </member>
          <member>
            <name>Customer</name>
            <value>
              <struct>
                <member>
                  <name>CustomerId</name>
                  <value>
                    <int>1234</int>
                  </value>
                </member>
                <member>
                  <name>CustomerDetails</name>
                  <value>
                    <struct>
                      <member>
                        <name>CustomerName</name>
                        <value>
                          <string>Acme</string>
                        </value>
                      </member>
                      <member>
                        <name>Region</name>
                        <value>
                          <struct>
                            <member>
                              <name>City</name>
                              <value>
                                <string>Somecity</string>
                              </value>
                            </member>
                            <member>
                              <name>State</name>
                              <value>
                                <string>Somestate</string>
                              </value>
                            </member>
                          </struct>
                        </value>
                      </member>
                    </struct>
                  </value>
                </member>
              </struct>
            </value>
          </member>
        </struct>
      </value>
    </member>
    <member>
      <name>S</name>
      <value>
        <struct>
          <member>
            <name>Serial</name>
            <value>
              <int>5678</int>
            </value>
          </member>
        </struct>
      </value>
    </member>
  </struct>
</value>

[awoodbeck]

I misunderstood the original issue to involve decoding responses. This PR may serve as a starting point for someone to implement the encoding functionality and restore symmetry. I'll close the PR but keep the branch around since I may not have time for at least a week or two.

[icholy]

Marshaling embedded structs should probably work like encoding/json and flatten them out: https://play.golang.org/p/nBgb6jemFTd