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!
This should maybe be `if @logged_in.tasks.any?` and probably really `if @logged_in.tasks.incomplete.any?`.

I'd like for us to explore caching this count on the person model so an additional query isn't needed every time the layout is rendered. Perhaps we can use [counter_cache](http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to) or a [custom callback](http://stackoverflow.com/a/24634425/197498) to increment an `incomplete_task_count`.