Wrap rake db:seed in a transaction.

This gives about a 30% speedup on my machine (from 30s to 20s). See
http://stackoverflow.com/questions/1711631/how-do-i-improve-the-performance-of-sqlite
Insist that crops created by db:seed are new.

This shaves 2s off the 22s runtime of `rake db:seed` on my machine.
Merge pull request #343 from pozorvlak/seed-transaction

Wrap rake db:seed in a transaction.