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 use `FactoryGirl` for the spec side of things, what about using it here?

```ruby
FactoryGirl.create_list(:venue, 100)
```

I think if we go that route your use of `Faker` here should get moved to the factories. [Currently the factories](https://github.com/calagator/calagator/blob/50375d1b20b675d0fcf8791e5be187d6a76c0b79/spec/factories.rb#L2) are building out attributes manually.