Shared extensions

This PR does away with the internal and linked extension methods in Elasticsearch.Net and NEST by making use of the new Shared Project type that was introduced in VS2013 Update 2.

The solution now contains a Shared folder, and in there is the new Shared.Extensions project.  I'm open to suggestions on a better name/structure.  Also, note that Shared.Extensions only needs to be added as a reference to Elasticsearch.Net, since it's at the lowest-level and is referenced by everything else.

Make sure you install Update 2 as well as the Shared Project Reference Manager extension: http://visualstudiogallery.msdn.microsoft.com/315c13a7-2787-4f57-bdf7-adae6ed54450

Closes #717