NCBC-265: Refactor solution to include directly the Enyim dependencies

The Enyim.Caching submodule has been removed, and the
Enyim projects have been moved into the couchbase-net-client
source structure.  Appropriate 4.0 and 3.5 versions exist
for each of the Enyim projects.

Solution references have been updated, and NuGet dependencies
are now set to download when the project is built, rather
than being stored in the repository.

Change-Id: I6e137d30ac18284a7168a7633c4f86af1d86cf39