[WIP] 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
- [ ] Allow to register a visitor

Screencast:
![registration](https://cloud.githubusercontent.com/assets/2076632/2978053/cad7e800-dbb3-11e3-8aa4-fc9810e39e62.gif)