Package: add each_file helper function
Fedora: Fetch object ID from METS
Improve checksum helper function, move to utils
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 new File model
Add post-store AIP callbacks
Update requirements
Callback: extract for package.each_file
Package.each_file: fix package name
Package.each_file: fix joining path
Callback: return success/fail counts
Callback: send all responses as JSON
Callback: only return JSON on failure