Prevent Choking on Pseudo Classes/Elements. Fixes #17
Rather than let the NotImplementedException bubble from CsQuery, we strip out pseudo classes/elements before processing. We also add a HTML comment to the output to say we've done so to (hopefully) avoid confusion.

Also realised I've been writing 'psuedo' everywhere. So corrected the typos too. <facepalm/>
Merge pull request #26 from robcthegeek/issue-17-psuedoelems

Prevent Choking on Pseudo Classes/Elements. Fixes #17