decouple from mongo

[facundoolano]

Make the code generic enough so a new db backend can be implemented without having to modify the invisible code.

[facundoolano]

An interesting way of doing this would be to let the different db backends define what operations can be done with the invisible models. For example, mongo might define find and findById, while elasticsearch would have things like search and suggest.

We could then have packages like invisible-mongo and invisible-elasticsearch, and this one would be just in charge of generating the proper REST endpoints and exposing to the client whatever API the db backend defines.