Add seeds

Event and Venue seed data should now be available locally via `rake db:seed`. Remaining issues are these:

- Should Event have an :updated_at value? This seems like it's automatically created/updated. Is that correct?
- Geocoding for seed data currently doesn't work. Does this matter? It appears to be related to geocoding being based partially on valid addresses rather than lat/long. Faker addresses are obviously not valid.

We can use `Array#sample` here.