Remove the XSS filter.

The xss() function was originally a port of the XSS filter from
CodeIgniter. I added it to the library because there wasn't an
alternative at the time. Unfortunately I don't have the time or
expertise to maintain the XSS filter or keep merging upstream
changes.

If you need one for your app, I suggest looking at Caja sanitisation
engine maintained by Google. (https://code.google.com/p/google-caja/
source/browse/trunk/src/com/google/caja/plugin/html-sanitizer.js)

Closes #123, #138, #181, #206, #210, #221, #223, #226, #227, #231, #232