Users can search for charities by town

https://trello.com/c/mYtQ6N8d/4-as-a-user-i-can-use-a-form-to-find-a-charity-that-will-be-in-my-town-sometime-soon
![filter_towns](https://cloud.githubusercontent.com/assets/8646429/5096584/643e81fe-6f41-11e4-8861-1940d4ccf12c.gif)

Unused block argument - `key`. If it's necessary, use `_` or `_key` as an argument name to indicate that it won't be used.