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
what about adding registry for fixtures? Default value would be `None` and later we could use `fixture_modules = packages or registry.fixtures`.