Bitlbee patches

[yourealwaysbe]

This is a collection of changes made so that things are usable with bitlbee. In particular

• store session data in password using bitlbee callback (bitlbee does not store other account variables),
• add config option to ignore messages over a certain age (bitlbee does not remember the last received message),
• ensure buddies are in your buddy list when a message is received (history messages seem to arrive before buddy list data),
• use pidgin group chats to implement whatsapp group chats.

[hoehermann]

Thank you. I want to read through the changes, then merge. Sorry I did not manage to do so the last month already (with the old PR).

[yourealwaysbe]

Sorry I did not manage to do so the last month already

No problems, there's no rush :)