Implemented the HttpClient version (.Net 4.5) of the IConnection interface.

Full implementation, with the exception of the semaphore blocking on async requests.

Project was changed from .Net 4 to .Net 4.5 to take advantage of async/await. A nuget package for this should be generated with net45 as the dependency. This could technically be back ported to .Net 4.0 with the async targeting pack and the net40 HttpClient, I just didn't see the advantage.

I would love to see a side by side comparison of the perf between the two, will probably write one of those later once I get a bit more comfortable in the testing environment setup for this project. I definately have a few issues, as I had about 130ish failed tests due to not having my test environment setup properly (guide?)

Let me know if you need anything else added to the code.