Common friends count in suggestion list

Hi,

This simple change adds the possibility to show common friendships count in the suggestions list.

* Modified the Cypher query to count friendship similarities
* Added a commonFriends property to the user model

NB: I'm not really familiar with Slim and Mustache, so I would prefer to pass the commonFriends to the view rather than setting it as User property in the model, feel free to push against the P.R.

Chris