Added a checkstyle reporter, addressing issue #11

I was unable to find an official example of the checkstyle format, so I based it off of the format used in this commit:

https://github.com/stubbornella/csslint/commit/4af36f14d545575de60ede9e54c6d022a2936e3c

Please correct me if you find something more definitive that I can base the checkstyle XML on.