Handle malformed html sent by bot modules

- Stops malformed html from crashing the sleekxmpp client.
Much more stringent checking for send_message

- If package methods return a dict with mbody or mhtml keywords it will
  no longer crash.
- Ensure malform HTML that was attempted to be sent will not completely
  crash the bot.
- Explicitly defined the rules for dealing with these three arguments
  and have them documented.
Changelog.