Refactoring digital shipping calculation and display

* removed the deface override that modified the checkout/_delivery partial
* created custom digital delivery calculator
* removed digital_shipping_method from Spree::Order. Moved logic to available_shipping_methods monkey patch

The main goal was to eliminate the shipping method detection logic from the _delivery partial.
This was a bit of a hack all along and didn't really present the correct information to the underlying
order logic (i.e. rate_hash.blank? would return true when there was no shipping options available).
This should be more future proof and make overriding another aspect of the delivery process easier.

spec/models/order_spec.rb

@@ -49,6 +49,20 @@ describe Spree::Order do
       order.digital?.should be_false
     end
   end
+
+  context "digital shipping" do
+    before do
+      # TODO create digital shipping factory
+    end
+
+    it "should only offer digital shipping if all items are digital" do
+      
+    end
+
+    it "should not offer digital shipping if only some items are digital" do
+      
+    end
+  end
   
 end