building out unit tests

added valid test cases for style attributes/tags, anchor tags, and
images with data URIs
added unit tests to detect sanitization of embed, object, applet,
frameset, and frame tags
unit tests for head, link, meta tag removal
added double-encoding workaround with unit test

This entailed adding a System.Web reference to use
HttpUtility.HtmlDecode on the HTML before it is loaded by the
HtmlAgilityPack