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)
That would be the whole point. When a trainer leaves a club, he wouldn't take all his clients with him, right? So you simply delete the trainer in the first club (and all clients get their trainer sets updated), and create a new trainer in the next club which won't have any clients associated by construction.