otr.py 1.5.0: many changes

All changes:
- handle nicks containing unicode characters
- add /otr fingerprint command for showing fingerprints
- disable send_tag for ChanServ, MemoServ and NickServ
- end OTR sessions when their buffer is closed
- colorize a lot of output that was previously not colored
- user configurable prefix for messages from script
- handle incoming IRC ACTION in unencrypted messages
- only modify actions if the context is encrypted
- restore default log level after OTR conversation
- make PRIVMSG parsing case insensitive
- raise an exception when a PRIVMSG can't be parsed
- fix parsing PRIVMSGs where the message doesn't start with ":".