Add a invoked_method parameter to RTMP.process_packets().

This makes it possible to wait until a method is invoked
by the server and then stop processing packets.
rtmp.RTMP: Replace conn argument with connect_data.

This value generates "conn" options automatically
from a Python value.
Raise RTMPTimeoutError on timeout.
rtmp.RTMP: Add more info about buffer argument.
rtmp.RTMP: Rename @remote_call to @invoke_handler and add remote_method().

remote_method returns a Python function that will
attempt to call a method on the server.
Send the return value from invoke handlers back as a _result.