Added support for Silverlight 5.

A number of changes:

* Added Silverlight 5 to the list of targets required by Refit-Portable.
* Added a Silverlight 5 project.
* Added a sl5 directory in ext, with Silverlight 5 versions of
dependencies (Castle.Core, Async Targeting Pack, Newtonsoft.Json,
Reactive Core and Interfaces).
* WP8 project no longer defines SILVERLIGHT.
* Liberal use of SILVERLIGHT #ifdef in HttpUtility to leverage
System.Windows.Browser.HttpUtility support for encoding.
* Changed HttpEncoder to store entities in a sorted list, rather than a
SortedDictionary not supported by SL 5.
Some cleanup?
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?
Made Silverlight project independent.

SL5 project is now independent of the Portable project.
Merge remote-tracking branch 'origin/pr/10'