Adding first version of Dashboard backend

First step for dashboard feature: "simple statistics"
* number of apps per user
* number of total devices of user owned applications (and their variants)
* total number of messages sent for the user's applications

This is reflected in the "top part" of the first screen:
https://raw.githubusercontent.com/hbons/aerogear-design/master/Unified%20Push%20Server/activity-spec.png