Add and document abstract _migrate method
Minor verbosity fix
Change back-references as needed on primary key change.
Update back refs on reload
Merge pull request #58 from sloria/print-function

Use print function
Merge branch 'develop' of https://github.com/CenterForOpenScience/modular-odm into abstract-migrate
Fix PK changes; update and test foreign queries

Update foreign references to records with changed PK
Improve test coverage for PK change and fix bugs
Add _process_query method to StoredObject to handle foreign, abstract values
Test queries by foreign fields, abstract foreign fields
Merge branch 'develop' of https://github.com/CenterForOpenScience/modular-odm into develop
Merge pull request #56 from sloria/abstract-migrate

Add and document abstract _migrate method