- add a new Greenlet class that has features of Proc but a bit simpler and derives from greenlet
- its interface is more in line with Thread and Process from std lib: it has join() and get() instead of wait()
- gevent.spawn returns a Greenlet
- spawn_link, spawn_link_value and spawn_link_exception are available from gevent top level package
- so are joinall() and killall() methods
- greenlet.py split into hub.py, timeout.py, greenlet.py, rawgreenlet.py
- Hub derives from greenlet now