Formatter: Checkstyle

Created checkstyle formatter based off of the example here: http://www.jshint.com/docs/config/

`eslint -f checkstyle foo.js` will not produce output that looks like this:

<?xml version="1.0" encoding="utf-8"?>
<checkstyle version="4.3">
	<file name="foo.js">
		<error line="5" column="10" severity="error" message="Unexpected foo." />
	</file>
</checkstyle>

Fixes #14