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
Corrected a regression in xmpp
Presence works in tox
Hooked up the presence events from XMPP
Test & document setting custom headers/status codes in webhooks
Docs: Explain how to use templates with self.send()
Docs: Don't generate apidoc for tox backend for now

It's quite a pain to build it, don't want to scare away potential
contributors by making it difficult to get the necessary dependencies to
build the docs
Added presence to the test backend
pass on presence + status messages

- added presence to test backend
- added status messages on tox and xmpp
make PEP8 happy
Merge pull request #278 from gbin/presence

Presence API
Docs: Remove network mention from intro, mention tox on backends list
Docs: Add presence section to plugin guide
Document the various callback_ methods in the same style
Add docstrings to the Message() class
regression in the text backend