Users can create new surveys

https://trello.com/c/jDAp5xy0/1-users-can-create-a-new-survey

![screenshot from 2014-11-10 11 27 08](https://cloud.githubusercontent.com/assets/8154429/4979354/7a61b1e6-68f6-11e4-844f-a1ee1bf70226.png)

![screenshot from 2014-11-10 11 27 26](https://cloud.githubusercontent.com/assets/8154429/4979362/892f79e2-68f6-11e4-8cfd-bd47f878acd3.png)


`null: false` doesn't do anything with `add_index`. You can see that by looking in db/schema.rb