Separation of the client and store concerns (major change)

[bcogrel]

This PR introduce an important separation of concerns between the client and the store part. The two sides have now distinct models. The main (mid-term) objective is to allow a client model to interact with multiple (heterogeneous) stores.

Because the models differ on the client and store sides, Resources attribute values have to be converted when passing from one side to the other (e.g. during the get() and save() operations). These conversions are made by ModelConverters that are defined for each pair client/store models.

The available converters only deal with direct mapping. The current client models are usually imported from the store models. In the future, more complex converters should be introduced for supporting better data integration scenarii.

See https://github.com/oldm/OldMan/blob/master/examples/quickstart.py for an example of the new API.

Main changes:

1. Each of them have their own Models and ModelManager
2. The ResourceManager is now a ClientResourceManager and only concerns the client part. Some of its code have been delegated to the ModelManager.
3. ModelConversionManager and ModelConverter introduced.
4. Resource has now two sub-classes: StoreResource and ClientResource.
5. Model is extended by the ClientModel subclass. Models are now independent of the data store.
6. ModelManager (used by the DataStore and the ClientResourceManager) is extended by the ClientModelManager sub-class.

[coveralls]

Changes Unknown when pulling bfc3e1d on new-resource-manager into * on master*.