Draw and Fill on BitmapContext

[zgabi]

This PR allows the users to draw and fill lines/shapes on the BitmapContext object without require to have a WriteableBitmap object.

[zgabi]

I think it is easier to see the differences if you compare with ignoring the whitespace differences. There were indent issues sometimes, mostly between in conditional compile directives (#IF SILERLIGHT, etc)

https://github.com/reneschulte/WriteableBitmapEx/commit/1a6d003e5dca7f2ea25f13aa68cdf20d63aca2e7?w=1

[reneschulte]

This won't merge, please update to the latest base.