feature : add keyboard shortcuts

+ decentralized shortcut declaration
+ each service/controller is now responsible for declaring its shorcuts
- documentation (cheatsheet) is still to be maintained manually
- init order matters (shortcutService has to be instanciated before
  everyone else) => should have a standalone KeyboardService singleton
  which is ready as soon as it is loaded

js/service/HistoryService.js

@@ -7,8 +7,9 @@
   ns.HistoryService.prototype.init = function () {
 
     $.subscribe(Events.TOOL_RELEASED, this.saveState.bind(this));
-    $.subscribe(Events.UNDO, this.undo.bind(this));
-    $.subscribe(Events.REDO, this.redo.bind(this));
+
+    pskl.app.shortcutService.addShortcut('ctrl+Z', this.undo.bind(this));
+    pskl.app.shortcutService.addShortcut('ctrl+Y', this.redo.bind(this));
   };
 
   ns.HistoryService.prototype.saveState = function () {