Async requests

[dmpayton]

I'm coming around to the idea that async is the future of Python, and I'd like to support this in python-montage.

The APIRequestor class encapsulates the actual HTTP requests, so it should be possible to implement async functionality in custom requestors. These requestors should live in a montage.async submodule, and Client.__init__ should be updated to accept a custom requestor. There are a number of efforts to make the requests library async, so much of the hard work has been done for us.

1. AsyncIORequestor (asyncio)
   https://stackoverflow.com/questions/22190403/how-could-i-use-requests-in-asyncio
2. FuturesRequestor (concurrent.futures)
   https://pypi.python.org/pypi/requests-futures
3. GeventRequestor (gevent)
   https://pypi.python.org/pypi/grequests
4. TornadoRequestor (Tornado)
   https://pypi.python.org/pypi/trequests
5. TwistedRequestor (Twisted)
   https://pypi.python.org/pypi/txrequests