Add validations
Merge branch 'master' of github.com:supherman/spree_sales

* 'master' of github.com:supherman/spree_sales:
  [WIP] sales admin views
  Delete unused views and controller
  Simplify sales associations
  Fix issue with database cleaner
  Add support for sales with fixed price

Conflicts:
	app/models/spree/sale.rb
	app/views/spree/admin/sales/index.html.erb
	app/views/spree/admin/sales/new.html.erb