Add File, Callback models
Add Callback forms, views
Fedora: record each file to the database

In the absense of persistent UUIDs originating in the storage service,
storing the checksum and source ID allows the file to be recognized
when it comes back around again.
Add post-store AIP callbacks

Conflicts:
	storage_service/locations/api/resources.py
Space: update_package_status must always return a tuple
Post-store callback: always continue when checksum missing
SWORD helpers: log exceptions during download
SWORD helpers: checksum correct path
SWORD: track object_id at item, not root, level

The location where this is needed only has access to individual item
records, not the root level.
Post-store: filter on non-stored files, handle multiple
SWORD helpers: check for transfer before approving
SWORD helpers: name Fedora METS after object ID
SWORD API: store all METS files

Previously, only the first METS file for a given set of files would be
stored. This stores every METS file, so that multi-file transfers will
contain the full set of metadata.
SWORD: download MODS records with OBJ items