Allow more Trello settings

This is how the authorization screen looks like withe current social auth.
It says "Let **An unknown application** use your account?" and the auth token expires after 30 days (which I thought was anoying).

![before](https://cloud.githubusercontent.com/assets/385177/4285577/4a08840a-3d88-11e4-8843-e59265d0942e.png)

I added two optional settings:
```
SOCIAL_AUTH_TRELLO_APP_NAME = 'Best App Ever'
SOCIAL_AUTH_TRELLO_EXPIRATION = 'never'
```

-->

![after](https://cloud.githubusercontent.com/assets/385177/4285576/49eeeb44-3d88-11e4-8a92-b1fec6f83f93.png)