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.
Eng guidelines say "The namespace of the extension method class should generally be the namespace that represents the functionality of the extension method, as opposed to the namespace of the target type."