Feature #43 - relative dates: +3d, -2w

- Now dates (due, wait, limit) are parsed first as Durations, and on
  error reparsed as Date.  When a Duration is found instead of a Date,
  the Duration is added to the current date/time.

src/report.cpp

@@ -2365,7 +2365,7 @@ int handleReportStats (std::string &outs)
       }
 
       if (it->getStatus () == Task::pending)
-        daysPending += (now - entry) / 86400.0;
+        daysPending += (now.toEpoch () - entry) / 86400.0;
 
       descLength += it->get ("description").length ();