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)
Actually, I think the consistency problem might not arise since we'll be accessing the trainer lists through the containing club (which should have immediate consistency since we're going through the same club entity).

I felt it was kind of weird to be accessing clients through trainers without entering the club, but I guess that's just how the NDB model works when you can't change owners.