Account creation

This pull-request adds the possibility to create an account.

The workflow is as follow:
- User enters username, fullname, email and password
- Checks if username and email are unique
- Account is created with a password_token, email is sent to the email address provided
- User visit the link provided in the email
- password_token field reset to None
- User allowed to login
We have a token generator function already in fas.security.generate_token.
You might need to extend it to support sizes.