Dot net 45

[AnthonySteele]

The big one as planned, touching a lot of files.

• Please() and Response() return Task<T> not T. All requests are async
• using System.Net.Http.HttpClient (new in .Net 4.5) for the underlying Http connection. It's better in many ways. And it's async only.
• All tests that make api requests are async and await.
• Delete the HttpMethod enum as we can use the matching one in .net 4.5. But this is a class not an enum - each http method is now an object not an enum member so there are minor changes to tests and implementation to cope with this.

Tests in this solution are passing. Downstream tests may fail as the return type from the api wrapper has changed to an awaitable wrapper over the previously returned object. These tests may have to be fixed after merging.

[gregsochanik]

Looks good, my only issue is the internal tests will have to be fixed retrospectively before we know that everything's OK, but if all the main tests pass then there's no reason why these shouldn't once changed to deal with an async response in the same way.