Open invitation setting

Implementation of #64 issue:

![2014-02-13 17-16-33 sign up drone io](https://f.cloud.github.com/assets/365089/2159200/5713c434-94a0-11e3-94ed-98310e16852b.png)

Todo:

 * [ ] Fix html in templates
 * [ ] Add migration script for `open_invitation` setting in database