Silverlight 5 Support

This is a pull request for issue #5. (Also my 1st pull request so please let me know if I've screwed anything up!)

Have decided to target SL5 at minimum, since Refit is targeting the most recent versions of the desktop and phone platforms.

There was a little weirdness with getting the platform-specific RequestBuilderFactory to load; the Refit-Portable assembly version is set to 1.0.0.0; the code that replaces the class/assembly name doesn't change the version also; so when trying to look for SL5 assembly version 1.0.0.0 it couldn't be found as 0.0.0.0 was already loaded. Strangely, this doesn't seem to be an issue with the .Net 4.5 side where the platform-specific assembly is also 0.0.0.0... something I have forgotten about the difference between assembly loading in the Silverlight and desktop CLRs?

Have tested with a simple example; an ASP.Net Web API back-end controller that takes a parameter via query string, and returns a string. Can test more extensively if required before merging.