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?