- 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

instead remove 'hub' from _threadlocal when greenlet dies, so it will be restored by get_hub()
also, pass exit code of event_dispatch to DispatchExit instead of text message

- also use queue.Queue instead of coros.Queue

- when notifying links, do not reschedule the rest of the links in case of
  an error; just print the traceback since that what the parent will do anyway

- now None (the default) means raise self, False means "silent" interrupt;
  True does not mean anything.