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.
Don't need HttpResponse.WriteAsync(byte[], ...) or HttpResponse.WriteAsync(ArraySegment<byte>, ...) methods. Those are redundant with HttpResponse.Body.WriteAsync.