Bug fix: Add support for Microsoft downlevel-revealed conditional comments to the HTML parser. Addresses Issue #36.

Bug fix: Changed the way CDATA elements were being handled internally. Rather than being a System.Xml.Linq.XComment, a CDATA element is now a System.Xml.Linq.XCData.
Bug fix: Correct the way comments were being parsed. Prior to this fix, comments were actually doubly commented. (That is, <!-- !-- comment --> -->
Added unit tests for modified functionality