don't allow to have several ssl_error / fingerprint_error / plain_connection / insecure_ssl dialog at the same time for one account. Fixes #4642

ID will be autogenerated (like from): Done by dispatcher as it wraps transports send, when plugged into NonBlockingClient.

detection code

Don't change sleeper state when we send a presence to a user, to not think global status has changed from invisible. see #4631

Introduce a get_instance factory method for all plugins and other xmpp related classes. This will help us to decouple plugs in order to make them testable.

Switch to SSL version 3 as the channel encryption protocol. As of http://docs.python.org/dev/library/ssl.html this is the maximally compatible SSL variant.