Implement DB migration

[turnerhayes]

Mongo schema changes can break the app. At the moment, with no real usage, I address that by simply blowing away the existing data when I do a new release (and can remember to do it), but that obviously won't scale long term. We should have some sort of DB migration mechanism.

Possible options to look into:

• https://github.com/tj/node-migrate
• https://github.com/buunguyen/mongoose-migrate
  • Fork of the above with a couple of changes; first result when I googled "mongoose migrations" though
• https://github.com/afloyd/mongo-migrate