[#2844] Added an API to get the total number of billable and overhead hours.

* Added Issue#billable_time_spent
* Added Issue#overhead_time_spent
* Added specs

lib/overhead_issue_patch.rb

@@ -0,0 +1,38 @@
+require_dependency 'issue'
+
+module OverheadIssuePatch
+  def self.included(base)
+    base.send(:include, InstanceMethods)
+
+    base.class_eval do
+      unloadable
+    end
+  end
+
+  module InstanceMethods
+    def billable_time_spent
+      time_entry_hours_based_on_billable_flag(true)
+    end
+
+    def overhead_time_spent
+      time_entry_hours_based_on_billable_flag(false)
+    end
+
+    private
+    # Totals time_entries that are billable (true) or overhead (false)
+    # and returns the rounded value
+    def time_entry_hours_based_on_billable_flag(billable_flag=true)
+      time = time_entries.inject(0.0) {|sum, time_entry|
+        sum += time_entry.hours if time_entry.billable? == billable_flag
+        sum
+      }
+      if time > 0
+        return time.round(1)
+      else
+        return 0
+      end
+
+    end
+  end
+end
+