JUnit reporter

[eyelidlessness]

Addresses #104. Provides a JUnit XML reporter.

Adapts work from the following, which deserve most of the credit:

• https://github.com/julias-shaw/speclj-junit (mentioned in #104)
• https://github.com/julias-shaw/speclj-junit/pull/3 (PR into the former, providing error output)

(But I feel this deserves to be in the main lib.)

Changes to those prior works include:

• Support for ClojureScript
  • Besides typical clj -> cljc porting, I also included a namespace for DOM-based XML generation, providing (only) the relevant portions of the clojure.data.xml library (I was surprised that such a thing didn't already exist for cljs/cljc)
• Produces output to *out* (as other reporters do)
• Includes output for pending specs
• Simplifies error output implementation from https://github.com/julias-shaw/speclj-junit/pull/3, which included some redundancy (hopefully; at least the specs pass, and a cursory attempt to exercise those code paths seems to validate it in real life)

[slagyr]

This is really cool. A junit reporter is handy. However it can (should) be packaged as a separate library. Note that none of the speclj source files were touched in this commit, a sign of good Open/Close Principle design. The Package Cohesion Principles would have this as a separate package since not all users will need it and shouldn't to pay the cost of extra dependencies and complexity.

The original library (https://github.com/julias-shaw/speclj-junit) is already deployed as a plugin. That's ideal. People who need the JUnit output can simply add it to their project deps.