[RFR] Add registration page

- [x] Add registration page
- [x] Handle single/group registration
- [x] Mandatory fields: `firstname`, `lastname`, `address`, `email`
- [x] Send result by mail
- [x] Limit group from 3 to 5 members
- [x] Allow to register as a visitor

Screencast:
![registration](https://cloud.githubusercontent.com/assets/2076632/2978329/3e42135c-dbb8-11e3-8bcc-52b44fa01b16.gif)