Elixr.sax

- Updated models having parent-child relationships to define cascade operation
as delete when parent object gets deleted.
- Introduced EntityWithDeletedMixin and updated models to derive from it so that
they have the deleted field.
- Added API functions implementing business logic for CRUD operations for models

- Added OrganizationType model and extended Organization

- Used UUIDs for all foreign key relationships
- Upgraded elixr.base to version 0.4

- Production release.

- Initial version.