Added Order Model
Remove item_ids column from Order
Added Payment association to Order
Updated order model with item_id removed
Made order_number unique and can't be null
Merge branch 'master' of https://github.com/spree-girls/spree-theme-market into add-order-model

Conflicts:
	db/schema.rb
Merge branch 'master' of https://github.com/spree-girls/spree-theme-market into add-order-model

Conflicts:
	db/schema.rb
Merge pull request #7 from spree-girls/add-order-model

Added Order Model