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.
This namespace needs to change. It should be in the same namespace as the Http abstractions