Fixtures exception on missing models

Functionality from https://github.com/inveniosoftware/invenio/pull/1974#issuecomment-49713726 which is useful even after resigning from the core functionality of the mentioned PR.

Includes:
- PEP257 improvements
- checking if every fixture to be loaded has a corresponding model
why not `models[fixture_name'] = getattr(module, model_name)`?