Fix FSharp.Net.Http in Silverlight
Remove Silverlight version

*The only reason we had a separate Silverlight version was because of the Freebase provider not working with the portable version in Silverlight, but I just tested it and it does now
*Also remove the designtime silverlight version. It's easy to add back later if needed, it's not even tested currently (closes #242)
Merge pull request #279 from ovatsus/master

Remove silverlight version