calculate all duration in the client

This cleans up some inconsistency where sometimes 'completed-started' happened
on the client side and sometimes on the server. Also should fix the 'cumulative
time' issue mentioned in #8

src/resources/js/app.js

@@ -330,7 +330,7 @@ var Job = function() {
             fillColor: "darkseagreen",
             strokeColor: "forestgreen",
             data: msg.recent.map(function(e) {
-              return e.duration;
+              return e.completed - e.started;
             }).reverse()
           }]
         }, {