Initial support for funding-goals-over-time; #141

This is for issue #141.

TODO:
 * Script for migration of current `goal` field data to `goals` table entry for each user
 * Edit schema to drop `goal` from `participant` table (safer after migration is complete)

I might be missing some places where goals are referenced. I tried to keep the API the same so hopefully it won't matter, but who knows, I've never used SQLAlchemy before.