Update lintin' wit' warnings we add'd in 0.6
Handle HTMLParseErrorr in MismatchedHTMLLintRule

* handle HTMLParseErrorr when tokenizin' strings in th'
  MismatchedHTMLLintRule so that we can output a neww W304 errorr when we
  see invalid HTML
* add skip_if decoratorr to test infrastructure so we can test one thin'
  in Python 2.6 and anotherr thin' in Python > 2.6
* add test fer Python 2.6 behaviorr
* add test fer Python > 2.6 behaviorr

Fixes #53