Silverlight fixes

* Adds a proxy to the silverlight demo so external urls work (#27)
* Removes old proxy code from freebase that is no longer needed
* Adds a design time assembly for silverlight so this can be used on TryFSharp.org
 * Uses the special sl5-compiler version of FSharp.Core
* Adds a runtime version of FSharp.Data for silverlight.
  * The portable version can still be used in Silverlight, but neither Freebase (because of System.Core.dll) or XmlProvider (because of System.Xml.Linq.dll) will work.
  * In the silverlight version Freebase works, but unfortunately XmlProvider continues not to work


Although in the silverlight version we're specifically using 5.0.5.0 version on the generated expressions, and not a retargettable portable version, for some weird reason the F# compiler inserts a reference to 4.0.0.0 in the assemblies that use the type provider. Seems like a weird bug maybe coming from the fact that the compiler uses 4.0.0.0 version internally?