restructuring unit tests
adding ExCSS NuGet package

We will need CSS parsing to detect @import directives and other such
potential injection vectors
refactoring work and expansion

HtmlSanitizer now behaves as a static class.
Additional detection of @import and expression directives in CSS are now
excluded.
Certain MIME types are now always blacklisted.
Frames, framesets, layers, ilayers, applets, and base tags are now
always blacklisted.