Issue/17 consistency

changed the models for strong consistency, which means every entity has a direct parent, except for two root models (User and Club)

_implication_: the resources should be adressed with the whole ancestor path (i.e. /trainers/1/clients/2/phases/3/circuit/4/exercises/5, or /clubs/1/trainers/)

transfered the models to use the NDB api from google-appengine instead of DB (always stay cutting edge!)

also added some documentation for the API (with examples)
We might still have consistency problem because the Trainer belong to a User, so they will be in a different entity group. But as @nsanglar said, it'll be rare so I don't think it's an issue for now.

It is indeed strange to have immutable parents... but we have to work with it for the moment. Eventually we can have our own DB.