Uniformize api responses

Now the rest service is self-discovering, each entity return links to
further explore the entity or to edit it.
Add a module response_producer which helps us in that way.
Also removed all functions get_representation from entities.
We don’t need them anymore as responses are produced by the
response_producer’s functions
Added an entry point to the api.

Added an handler method ‘/api/enter’ which will serve as the entry
point for the api. Will be used to return links and initialization
information for any client