Provide synchronous API

[Jajcus]

The asynchronous API is efficient and flexible, but coding would be simpler with some synchronous API, with no need for callbacks defined by an application.

A code using the synchronous API would look something like this:

client.connect_s()
print("Connected!")
client.send(some_stanza)
peer_version = request_software_version_s(client, client.stream.peer)
print("{0} version: {1}".format(client.stream.peer, peer_version))
client.disconnect_s()

Some ideas for implementation:

• for the threads running the main loop synchronous calls would be implemented by recursive calls to mainloop.loop(). A special 'SyncMethodExit' exception could be used to exit such recursive loop calls
• other threads would just wait for some event
• a generic wrapper could be made for asynchronous calls taking a callback and error_callback arguments. The wrapper would require access to the main loop. In case of calls which take a Client instance as the first argument client.loop could be used. Or maybe we should use thread-local storage to pass the current loop object?