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'm not sure moving this is a good idea. Now I have to add another assembly to WriteAsync with a string. We may have jumped the shark here.

Maybe we should reconsider putting these extension methods into the main assembly /cc @lodejard