gevent.signal: spawn Greenlet instead of raw greenlet; switch into the greenlet immediatelly when the signal handler is called

start() no longer raises an exception if Greenlet was already started. it also now does nothing if the greenlet already exited (previously it would schedule another switch)

Hub: after LoopExit was raised, it was possible to switch into dead Hub; fix it to re-enter loop.run() and then raise LoopExit in a loop

--HG--
rename : examples/webchat/run.py => examples/webchat/run_standalone.py

resolver_ares.py:
- gethostbyname now handles '' (empty string)
- getaddrinfo now handles integer ports of type string, e.g "25". Thanks to kconor.
- getaddrinfo now converts UnicodeEncodeError into error('Int or String expected')
- getaddrinfo now uses the lowest 16 bits of passed port integer, to mimic socketmodule.c
- getnameinfo calls getaddrinfo to process arguments, similar to socketmodule.c
- gethostbyaddr also uses getaddrinfo to process arguments

ares.pyx:
- added InvalidIP exception
- gethostbyaddr and getnameinfo now raise InvalidIP immediatelly instead of passing it through callback

"kill" is available as a deprecated alias

convert signal() into a class; the usage did not change; added property 'ref' that is forwarded to watcher

add "ref" argument and property to all watchers; setting it to False makes watcher call ev_unref appropriatelly

removed _incref member
added _flags member