Account XMPP connections

[sleduc]

As for now, if we want a bot to join several rooms, pipobot will create as many XMPP clients as room it has to join.
We could improve this behaviour by:

• creating an 'account' configuration with a jid/password
• associating each room with a JID
• creating one XMPP client instance by account, and it will join several rooms if needed

To manage that we will obviously have to change how the BotJabber is handling connections, thanks to the muc plugin of sleekxmpp: https://github.com/fritzy/SleekXMPP/blob/develop/sleekxmpp/plugins/xep_0045.py
For instance the muc plugin has a "room" attribute where information about all joined rooms are stored.

This may require to change the organisation, configuration and communication between the bot and modules.