Create Spree::StoreHour model with tests
  * Create spree_store_hour table to manage store hours
  * explicitly require day
  * Add "for" class method to get store hours for a specific day of the week
  * implement logic to determine if the day is closed
  * format time appropriately for open and close time attributes (US 12-hour clock)
  * Implement model tests
  * Add timecop gem for testing
  * Add store hour factory