Code Cleanup Part 1.

Added ConfigureAwait where needed. Made a few classes internal. Added
the unit test assmebly as a friend to RestClient.NET.  Made
IRestDataSerializer and RestClientBase both Disposable.  Code coverage
is now at about 85%... However, Most of the tests aren't really hitting
edge cases. The code still lacks commenting where needed. My next commit
will be mostly additional unit tests and code comments.