run fetches in parallel

[oconnor663]

We'll use asyncio for this, from the 3.3-compatible "tulip" library. Some things to remember:

• Make sure modules and rules lock their get_tree methods. We don't want to allow multiple fetches to happen at once for the same module.
• Use a semaphore to limit the number of parallel jobs.
• Refactor resolver.py so that not everything needs to become a coroutine.

[oconnor663]

Tests passing: https://phabricator.buildinspace.com/D58

[oconnor663]

--parallel flag to control parallelism: https://phabricator.buildinspace.com/D59

[oconnor663]

Stackoverflow question to help with testing this: http://stackoverflow.com/questions/25389895/how-can-i-tell-when-the-event-loop-is-idle

Edit: Really poorly presented question. I deleted it. But the closest thing to what I was looking for is BaseEventLoop._run_once(). It's an implementation detail, but maybe it wouldn't be the end of the world to depend on it in a test.