Removed separate project directory for SL5.

Refit currently has all source in one directory.
Ensure the BCL Build MSBuild targets are imported by SL5 and WP8.
Very basic example working in SL5.

* Have tested against a very simple example in SL5; /api/hello/{name}
returning a string. Worked.
* Had to include the AssemblyInfo.cs from the Refit-Portable project in
Refit-sl5 project. Otherwise, when the platform-specific
RequestBuildFactory is attempted to be loaded, it looks for version
1.0.0.0 when version 0.0.0.0 has been loaded. This doesn't appear to be
a problem on the .Net 4.5 side however?