Feature/bulk upload composite

# Bulk Upload Support of composite relationships

## New Features

- support of additional informations in the relationships csv files
- tests for that
- new method `iterate_model_fields` to replace `get_model_fields` that returns a generator
- `*/contribute/upload/` added to urls.py
- makefile : don't stop on error during the `syncdb` operation

## Next to Do

- [ ] Bulk upload: show better warning messages
- [ ] Bulk upload: allow to remove freshly added nodes
- [ ] Bulk upload: compress files at the job input