Preliminary support for TOX network

TOX is a decentralized, anonymous and encrypted chat network.

So far you can with this backend :
invite the bot
1 to 1 chat with the bot
invite the bot to a groupchat
chat in the room with the bot

At this point : no security (anybody can invite the bot)
Pass on TOX backend

- now the basic security is done (admin / not admin).
- message splitting too
- bootstrapping is configurable
Test regression on self.jid
made pep8 happy
Merge pull request #277 from gbin/tox

Support for TOX network