Security registration strategy

[Nek-]

For now, the registration is open to all. That's maybe what people wants but most part of the time it's not. The kind of registration should be configurable:

• Open registration;
• Registration with captcha (awesome if customizable via bundle conf), the default must be google one's and this must be the default registration;
• Disabled registration, that also mean we must create a command to create users.