Moving groups to the home page

- added uniqueness validations for membership
- added callback that adds owner to her own group
- added a list of groups on the home page

@zauerbach The rest is yours! There is a lot of javascript in `home#show` and `polls#show`. Hopefully they provide sufficient examples for what you intend to do. Feel free to clean up or remove any of these. If you are writing a lot of javascript, I recommend moving them to `assets/javascripts` (see `assets/javascripts/stream.js.coffee`.)

You might need to refer to the Faye [documentation](http://faye.jcoglan.com). Please feel free to approach me if you get stuck. Otherwise, I am looking forward to seeing your work on Wednesday.