Adding support for OData $select and $expand.

Only ODataFormatter and Json Formatter work with this $select and $expand
implementation. If you have [Queryable], $select and $expand should start
working for you automatically. Refer to SelectExpandTest.cs for some
samples.

With added support for $select and $expand, one can enable [Queryable] on
non-collections too. Use SingleResult<T> if you want to support $select
and $expand on single entities efficiently. Refer to
NonODataSelectExpandTestCustomersController for an example.