Mongoid and other useful stuff

This PR includes setup of base useful gems : 

* **mongoid** which is an ORM working on top of MongoDB. Therefore I removed ActiveRecord which is a SQL based ORM.
* **rspec** , a popular testing framework, instead of default Test::Unit
* **factory_girl** , an on-demand model objects factory, used in tests, instead of fixtures.
* **figaro**, that handles sensitive configuration data like encryption or API secrets.

We are now almost set for the config part.