Implement a registration service

[fredmaggiowski]

A registration service which exposes a REST API to perform basic user registration, service registration and fare billing plan.

A user should be identified by a username (primary key) and an email address (which must be unique).

During user registration phase there won't be any service subscription since the account must be activated before proceeding any further.

Also registration (at least in the first alpha) will be perfomed via invites, this mechanism must be implemented