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

multithreading is not supported by core.pyx yet

also, make core available as an attribute on gevent, so that the following works:
import gevent; gevent.core.something

- drop GreenFile (makefile() uses _fileobject), GreenPipe and GreenPipeSocket

- add greenlet.Waiter class (internal helper)