Add new manager for maintaining arbitrary ordering
Add `step` argument to bulk update method of OrderingManager
Make a sensible default for scan ordering
Use OrderingManager on models requiring arbitrary ordering

These are: scans, citations, and note sections
Refine API for OrderingManager

Also, fix bug, when positions_dict is empty but values should still be
filled in.
Add controller to API to normalize scan section order

This might not be the best solution, but it works for now.
Remove now-unused "note ordering" field
Bump rest_framework to 2.3.x
Add scans to API
Make sure changes are committed in order normalization method
Make sure int arg is an int

lol python
Add normalize_order controller for document scans

This should be limited to POSTs in the future
Add ability to make notes private

Both in model and in API
Make scans field read-only in document API
Merge pull request #224 from editorsnotes/REST-api