Cleaned up the html => image conversion

Hi Pablo,

Here are some changes which should make Dan's image classes ready for primetime.

1) Added a base interface IHtmlToXConverter (the specific image/pdf interfaces derive from it)
2) Created an abstract converter which contains most of the functionality of the old WkPdfToHtmlConverter
3) Made the MultiplexingConverter generic (renamed to GenericMultiplexingConverter<T>) so it handles both PDF's and images.

This eliminates most of the code duplication. I was thinking to generate the child classes WkPdfToHtmlConverter and WkPdfToImageConverter from a T4 template. (This is because the PInvokes are very similar). But I decided not to make things too complicated :)

Please let me know if you need anything further to merge this into the main branch.

Best,
Ismail