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)
Won't we have consistency problems when the club manager wants to delete a trainer? I'm not sure I'm convinced by trainers that aren't owned by the club.