First step in an async HTTP API.

This is very experimental right now but it sets us up
for a full asynchronous API. The current implementation
builds an async API over our existing synchronous API.
Future refactorings to the internals should promote the
async API throughout the codebase, particularly in SPDY.
That way we can have more requests in flight than threads
processing those requests.
Merge pull request #225 from square/jwilson/experimental

First step in an async HTTP API.