fd is available as event.fd

test__socket.py: add socket-related tests from api_test.py; make all of the tests run ssl as well as tcp

it does not seem possible that and event may need to be cancelled and DECREFed in __dealloc__

restore lazy creation of the Hub in get_hub() and the ability to replace greenlet.Hub with another class

proc.Proc.wait: simplify implementation and change signature a bit: exception instead of *throw_args

greentest/__init__.py: use greenlet and core accessible through gevent instead of importing them directly