Add close method to websocket client; fix socket leaks in websocket_test.

These leaks are detected only in python 3.4 thanks to its improved
generator GC.
Initial version of asyncio/tulip-based IOLoop.

Currently always creates a new asyncio event loop instead of using
an existing one.
Split AsyncIOLoop to make it possible to use the global asyncio event loop.

AsyncIOLoop works more like Tornado's native IOLoop so it's better for
our tests, but AsyncIOMainLoop is more appropriate for most real-world
usage.
Add a tox config for the py33 backport of asyncio.

Update some old comments.