- make sure link() is always async
 - add rawlink() that does not spawn a new greenlet per link
 - linking to greenlet does not spawn auxilary greenlet anymore
 - throw() method overriden to handle killing of not yet started greenlet properly
 - _schedule_run and _schedule_switch renamed to schedule_switch and schedule_switch_later
 - make spawn_later return tuple (greenlet, timer)
 - joinall - change order of arguments to joinall(greenlets, timeout, raise_error)

--HG--
rename : greentest/test__proc.py => greentest/test__greenlet.py
rename : greentest/test__proc_procset.py => greentest/test__greenletset.py
rename : greentest/test__proc_pool.py => greentest/test__pool.py

- 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