Specify optional `asyncio` requirement properly

Previously, the runtime configuration for this would not work with wheels, which are static archives (and their `setup.py` is only run at build time, so is only valid for the python version that `setup.py` was run with). This should fix that problem for wheels, i.e #25.

[This part of the wheel docs](http://wheel.readthedocs.org/en/latest/#defining-conditional-dependencies) explains conditional dependencies but for the *next* release of wheel, 0.24.0, not the current ATOW. The `[metadata]` section in `setup.cfg` will work to correctly mark the cond. dep till 0.24 is released.

websockets is not a universal wheel either, it relies on python 3.3+, so I changed `setup.cfg` to reflect this. [See this part of the wheels docs](http://wheel.readthedocs.org/en/latest/#defining-the-python-version)