Implement GET method in 'swagger_handler' in order to retrieve the JSON specification (swagger.json)

[cabol]

The idea here is implement a Cowboy swagger handler to retrieve the swagger.json. In this case (initial version) the JSON will be generated during each request (that could be improved later). So the handle function should be able to:

• Call trails:all/0 to get defined trails
• With these trails, call swagger_json:from_trails(Trails) to generate the JSON and then return it.