Secret URL to access unpublished competitions

Resolves issue #496 

## Functional Review

1) Create a competition
2) Don't publish it
3) Go to view the competition
4) There should be a "secret link" underneath the title
![image](https://cloud.githubusercontent.com/assets/2185159/3845341/7196dfcc-1e51-11e4-9f0b-41320d1e2e92.png)
5) Anyone should be able to see the competition with this secret URL, but should not be able to access the URL without the secret.

## Tasks

- [x] Can't access unpublished competition view without secret key
- [x] Secret URL in template for organizer to give out
- [x] Add model field for secret key

## Tests
- [x] Competition view returns 404 if not published 
- [x] Competition view returns 200 if not published, but correct secret key given
- [x] Competition view returns 404 if not published and incorrect key given
- [x] Competition view returns 200 if not published but you are the owner