Package: add new is_compressed helper
Package.extract_file can extract the whole package

Extract_file now takes an optional extract path which defaults to a
temp directory, and omitting the relative path extracts the whole
package.
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 post-store AIP callbacks
Update requirements
Fix get_file_checksum import
Add migration for File model
Package: add each_file helper function
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