Added support to generate a report file. (Issue #9)

I implemented JSHint compatible reporter classes, so please check it out! I love your jshint-mojo, thank you.

* Added 2 configurations, <reporter> and <reportFile>
* If following value is specified in <reporter>, JSHint report file is generated to the path located by <reportFile>.
  * jslint: equivalent option to --reporter=jslint
  * checkstyle: equivalent option to --reporter=checkstyle
* Implementation restriction
  * Custom reporter is not supported yet.
  * Although original JSHint's reporters are implemented by JavaScript, commited reporters were re-implemented by Java for Rhino version JSHint.