Include error codes in reports

Hello, i noticed that JSHint has a nice system of error codes and messages divided into **E**rror, **W**arning and **I**nfo. I wondered why it would not be included print out on the command line, so i first added it to the *default* reporter.

I also wanted to use this with my Jenkins setup, so i also added it as "severity" attribute to the *jslint_xml* reporter. I use this in conjunction with my patched Jenkins violations-plugin:
https://github.com/croensch/violations-plugin/commit/7f2c5457fad0745554cbed31c8c4dae588d29f85

Only thing missing is the *checkstyle* reporter now, as i had no need for it. The other two run fine in production. So let me hear how to move this forward.