Included the excellent MarkdownDeep parser for converting Markdown to HTML (with full support for GFM tables). I had to make minor changes to the MarkdownDeep source (remove support for sizing images, changed string.Compare to use alternative overload) so it would work within the constraints of Portable .NET code.
Removed MarkdownSharp implementation (it is superceded by MarkdownDeep), but I left the MarkdownToHtmlConverter.Transform method in, for backwards compatibility (it now calls MarkdownDeep's Transform method)