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)
I like the idea of have differents rights for differents path. Right now all these paths work as expected but there is no access level management.

And cloning is an option, but if we do that we have still have to remove all the references from client to trainer because they would be invalidated by the cloning (the old key will be invalid). It feels cleaner to me to change a reference rather than cloning an entity. It also means we would have to clone every descendant of that entity, which could be error-prone too.