[WIP] Use current user in models

closes #2058
- fixed apiContext as suggested in the issue
- added user to options object for models
- added api.users.register() for public registration
- changed models to use options.user for created_by, updated_by,
author_id and published_by
- added override to session model to avoid created_by and updated_by
values
- added user (id: 1) to tests
- added user (id: 1) for registration
- added user (id: 1) for import, fixtures and default settings

**ATTENTION:** Tests are failing because of a bookshelf bug that is fixed in 0.6.4. Updating bookshelf causes other problems and has to be investigated.