HTML5 Parser

This addresses #2 and #3 by using the html 5 parser from the [nu.validator](http://about.validator.nu/htmlparser/apidocs/) team. 

Benchmarks show pretty much the same parsing speed, but unclosed tags are not a problem anymore, html entities are resolved correctly, and fully-qualified DTDs appear to be discarded altogether.

For now I think we are okay ditching the DTDs and just always outputting an html5 style doctype.