Dot net 45

The big one, touching a  lot of files. Checkpoint commit not that 
- 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 only async.
- all tests are async and await.
- Delete the HttpMethod enum as .net 4.5 has this (though each http method is now an object not an enum member)