Feature/mass updater

Adds a class, script and two intro migrations. Class is MassUpdater and allows running a migration function on every document in a given models table. Migrations are python modules which have a docstring for the description, a `models` list and a function called `migration` which takes one argument and manipulates it in some way. No return statements are needed or used. The script, `migrate.py` facilitates running each of these migrations in an easy to use CLI interface.

This opens #24 and fixes #21 for phots for realz.