Basic user's action added to API

Introduced in the API basic user interaction with other NERDZ's entities like user and posts.
In fact, using the API is possible to enstablish a connection between a specific user *x* and another entity *y*, which could be or a project.
The user now is able to bookmark a posts which is present or in a user's board or in a project's board.

All the tests are executed on Postgres with a fresh and clean nerdz-test-db installation.

**Be careful** 
Actually we are **not** using the ORM library created by jinzhu (https://github.com/jinzhu/gorm) but a personal modified fork (https://github.com/galeone/gorm) until the pull-request will be accepted.