New models

Models done
I'm testing right now and it seems to work properly.

```rb
FactoryGirl.build(:task)
#<Task id: nil, task_plan_id: 5, title: "A task", opens_at: "2014-11-04 18:26:22", due_at: "2014-11-11 18:26:22", closes_at: "2014-11-18 18:26:22", taskings_count: 0, created_at: nil, updated_at: nil> 

FactoryGirl.build(:task, opens_at: Time.now + 1.week)
#<Task id: nil, task_plan_id: 6, title: "A task", opens_at: "2014-11-11 18:26:39", due_at: "2014-11-18 18:26:39", closes_at: "2014-11-25 18:26:39", taskings_count: 0, created_at: nil, updated_at: nil>
```