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)
You can easily deal with club changing by deleting the old trainer and cloning a new one into the new club. This also makes sense because you won't have to  find all users that are pointing onto the trainer and remove the references.

I'm also wondering whether we shouldn't have different access paths for clients:
1. /club/1/clients would give access to all clients in the club but can only be called by a manager-level user
2. /club/1/trainer/2/clients gives access to all clients of trainer 2 and has a trainer access level
3. /club/1/clients/2 gives direct access to clients and requires manager-level access OR being client 2

It doesn't make much sense to have to add the club manager to all client trainer sets as this would be pretty error-prone so it would be cleaner to have the different access paths with different access rights.

WDYT?