Added support for STI deliverables

* HourlyDeliverables cost are based on the time used
* FixedDeliverables are based on a flat rate
* Added a checkbox on the new Deliverable form to pick which one to use

  #1135

app/models/fixed_deliverable.rb

@@ -0,0 +1,3 @@
+class FixedDeliverable < Deliverable
+  unloadable
+end

app/models/hourly_deliverable.rb

@@ -0,0 +1,3 @@
+class HourlyDeliverable < Deliverable
+  unloadable
+end