Add response writing extensions.

#66.

Adds lots of WriteAsync extensions for simple writing to the response.

Adds lots of SendAsync extensions for sending an entire response, including setting Content-Type and Content-Length.

Note BOM's are not ever written out when using Encoding.Utf8.

We should consider moving GetFormAsync to this extensions package.
I don't think we intended to move the HttpResponse.WriteAsync(string) to an extension method in another assembly. Let's keep the WriteAsync(string, ...) methods in the Microsoft.AspNet.Http project and have the SendAsync methods in Microsoft.AspNet.Http.Extensions.