Added basic xml support.
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.
Forgot to set RestClientBase->Disposed to false

Forgot to set RestClientBase->Disposed to false in the private
constructor. In theory this probably wouldn't affect anything, but
explictly doing it is probably better.