Add task functionality to groups.

Implements the basics for issue #241.

![screen shot 2014-09-06 at 17 38 07](https://cloud.githubusercontent.com/assets/1248040/4175579/e24f12ba-35db-11e4-83ff-7abb915c2457.png)

I'm creating this PR to get your feedback on this functionality. I've tried to stick with AdminLTE styles as much as possible. Tasks are sortable and commentable. Tasks are viewable under a group or an overview of assigned tasks from multiple groups.

There's still a few thinks I'm working on right now:
- A button 'Archive finished tasks'
- Setting a param to include archived tasks in the index (seperate list).

Some would be nices/later additions:
- Attachments
- Task groups (per project)
- Assign to multiple persons

Let me know what you think!
You need to add `has_tasks` to the en.yml file [here](https://github.com/churchio/onebody/blob/master/config/locales/en.yml#L102) so the label will be translated.