Library should make asynchronous HTTP requests

[emmettbutler]

Use tornado.AsyncHTTPClient instead of requests to make everything faster

[emmettbutler]

Implemented callback support in the core, still need to add it to all of the endpoint functions.
Also planning on supporting running as a coroutine from inside a tornado server.
Progress can be found in the async branch

[emmettbutler]

Callback support added to all public library methods emmettbutler@e760184