Add Specializations of net::async_result.

[ricejasonf]

Both promise and future could integrate with the Networking TS by specializing net:async_result or asio::async_result. This would also simplify calls to Asio or Net async functions.

Having a future of an error_code is kind of boring though. Perhaps tools to resolve the buffer or something deserialized on success would be handy.

Functions for timers would be cool too.