Add pickup date

This additional date helps users to find the correct order if the time
between end and pickup varies a lot.

db/migrate/20160217152621_add_pickup_to_order.rb

@@ -0,0 +1,5 @@
+class AddPickupToOrder < ActiveRecord::Migration
+  def change
+    add_column :orders, :pickup, :date
+  end
+end

db/schema.rb

@@ -227,6 +227,7 @@ ActiveRecord::Schema.define(version: 20160217194036) do
     t.decimal  "foodcoop_result",                  precision: 8, scale: 2
     t.integer  "created_by_user_id", limit: 4
     t.datetime "boxfill"
+    t.date     "pickup"
     t.integer  "invoice_id"
   end