Add JUnit-style XML file output.

I modified grunt-contrib-qunit to write JUnit-style XML files via the [JUnit reporter for QUnit plugin](https://github.com/jquery/qunit-reporter-junit). This is handy for visually rendering QUnit test results in Jenkins.

I'm new to both Grunt and QUnit, so I have some questions about how to best follow these two projects' conventions:

- How to specify the XML output files from GruntFile.js?
  - Currently, I just set a custom output directory option and create an XML file for each source file/url.
  - I'd like to use Grunt's conventions, but it's not simple to map multiple QUnit HTML source files to a single XML destination file (The output from qunit-reporter-junit.js are complete XML documents that cannot simply be concatenated ).

- Also, to get XML output from QUnit test files multiple steps are required:
  1. Set the XML output directory in GruntFile.js
  2. Include qunit-reporter-junit.js in each QUnit HTML test file. (Files without the script will run QUnit tests, but results will be missing from XML output.)

Would it make sense to automatically inject the qunit-reporter-junit.js script if an XML output directory is specified?