Jsx support

Hello :-)

I know that the PR for this branch on `grunt-contrib-jshint` has been rejected but I wanted to let you know that I like your fork to bring the support for JSX,

I brought some changes: the React tools seem to ignore the transformation unless it have the `/** @jsx React.DOM */` comment and even if the file is .jsx so I fixed that you can both use `.js` or `jsx`.
I personally prefer to use `.js` because it works better with Browserify (you explicitely need to do `require("./foo/index.jsx")` otherwise..

BTW I think we should create `grunt-contrib-jsxhint` on NPM

Cheers