Users can create pickups for a charity availability

https://trello.com/c/dyH2YTAp/5-as-a-user-i-can-schedule-a-pickup

![user_create_pickup](https://cloud.githubusercontent.com/assets/8646429/5109777/9a9ab10a-6fe1-11e4-9816-e4b869773747.gif)

``` ruby
def associate_donor_for(email, pickup)
  donor = Donor.find_or_initialize_by(email: email) # avoid hitting the DB here
  donor.pickup = pickup # more direct than using the id
  donor.save
end
```