Added a test reporter for JUnit XML

Taken from https://github.com/chenkelmann/junit_xml_listener and slightly improved.
Activating is now just a matter of setting `testReportJUnitXml` to true.
This not only brings a very handy feature (hudson/jenkins support that syntax),
but gives more options for testing the SBT test infrastructure.
For instance checking that the test output is well segregated during a parallel
test run is just a matter of checking the report output.

Conflicts:
	main/src/main/scala/sbt/Keys.scala
Migrate JUnitXmlReport into an autoplugin.
Remove enabling key for JunitXmlReportPlugin.

This migrates JunitXmlReportPlugin to work the way we desire
new sbt features/plugins to work:

* Enabling the feature is having the plugin available.
* Disabling the feature is disabling the plugin
* All code/settings for the feature are isolated to the plugin.
Adding docs for the test report hooks.
Merge pull request #1278 from sbt/wip/junit-xml-reporter

Migrate JUnitXmlReporter into sbt as autoplugin