Implement logging

[dx]

Many users need this feature (like me). It should offer two formats, greppable plain text and xml/html.

[dx]

I've started to implement it with j0hn, to help him Dive into emesene tm). In less than an hour, we did an almost full rewrite of Log.py. It doesn't integrate with Conversation.py yet, but outputs a nice (and highly greppable, readable, etc) text log.

[fretn]

a history plugin ala the gaim one which outputs the last xx lines of the last conversation in a newly created conversation window would also be nice.

[dx]

done, r653