Admins create availabilities for their charity

https://trello.com/c/0qfz3zvZ/10-as-an-admin-i-can-create-availabilities-for-a-town-on-a-date

![image](https://cloud.githubusercontent.com/assets/8646429/5090022/179a56f4-6f0d-11e4-93ca-86144294908b.png)

Towns are definitely shared by charities, but (maybe my naming was off) I'm going more for a "list of towns" object that charities keep so they can track where they've been and where they're going. A charity has many towns, specific to where it goes.