Test for REST API

This PR aims to implement functional tests for the REST API.

References:
- http://williamdurand.fr/2012/08/02/rest-apis-with-symfony2-the-right-way/#testing

Notes:
- To tun the tests:`bin/phpunit -c app`
- When you get the following error: `PHP Fatal error:  Call to a member function find() on a non-object` on Handler/PageHandler.php on line 20 (public function setUp()) 
you need to add 

```
    public function setUp()
    {
        // other code
        $this->om->expects($this->any())
            ->method('getRepository')
            ->with($this->equalTo(static::PAGE_CLASS))
            ->will($this->returnValue($this->repository));
    }
```