Remove HTTP abstraction layer

The .NET Framework provides the classes in the System.Net.Http namespace as an abstraction layer for HTTP requests. Rather than create a new abstraction layer in the .NET SDK, the SDK should leverage the existing abstraction layer and extend it as necessary. For example, offline unit testing can be easily supported by extending HttpMessageHandler to return deterministic responses instead of sending the request to a server.

The classes in the Openstack.Common.Http namespace form this abstraction layer. However, the namespace itself will likely be retained as the home for classes extending the standard abstraction layer.