Introduce AbstractExtDirectService

The AbstractExtDirectService extends the AbstractCrudService and provides additional methods, which can be used to load/modify stores or load forms in a EXT client. All currently available services have been adapted to extend the new service.
Adapt Model annotation of all models.

Adapted the crud-methods, which are configured in the Model-annotations.
Adapt the webapp-archetype to the new services.
Merge pull request #31 from buehner/ext-direct-service

Introduce AbstractExtDirectCrudService