Improve comment about abstract sockets.
Move from greenlet to python-fibers.

The implementation of python-fibers is more recent and somewhat simpler that
greenlet's. Also it supports PyPy out of the box.

@saghul, thanks :)
Refactor Queue and Events.

Give them a more standard interface. Queue is modeled after Python's Queue, and
Events has been split up into a Python like Condition,  and a ConditionSet.