Various fixes

- Fixes https://github.com/martinb3/hubot-standup-ng/issues/1, you can ask the bot who is on a team, or get a list of teams
- Fixes https://github.com/martinb3/hubot-standup-ng/issues/2, you can ask the bot to delete a standup (will take email settings with it)
- Fixes https://github.com/martinb3/hubot-standup-ng/issues/3, there's now environment variables to turn on or off the various backends