Remove repr implementation
Fix doc for optimistic_insert
Add ensure_backrefs helper for consistency checks
Change query from list to single object
Merge branch 'storage-abc' into develop
Fix query default on Storage::remove()
Merge branch 'develop' of https://github.com/CenterForOpenScience/modular-odm into redis

Conflicts:
	modularodm/storage/base.py
Remove abstract implementation of repr

Wrongly assumed concrete implementations have a store attribute