INCOMPLETE implement mongodb as a second database engine backend.
Add a string shortcut to filter results based on permissions.
Generalize database engine logic so it can be hotswapped.
Add support for arbitrary engines and finalize tested support for MongoDB.
Fix .travis.yml
Fix .travis.yml (again)
Add mongo to the dependency list.
Log the delete results.
Try another delete workaround.
Fix syntax error.
Remove unnecessary logging, promisify delete code.
Run all tests through all databases by default.
Merge branch 'mongo' into dev

Conflicts:
	test/permissions.test.js