removed tradable boolean field

we're now relying just on the "tradable_to" field, which is clearer.
added numeric validation to qty for planting/seed
tidying up/bugfixing seed display
Merge branch 'dev' into seed_trade
test cleanups (no idea how they got into that state)
set 'nowhere' as default seed.tradable_to
rake task to set seed.tradable_to to nowhere
fixed javascript datepicker for seeds
added slug for seeds
allow nil/blank qtys for plantings/seeds
Merge pull request #256 from Skud/seed_trade

Seed trade