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/keyboard/KeycodeTranslator.js

@@ -1,6 +1,9 @@
 (function () {
   var specialKeys = {
-    191 : "?"
+    191 : "?",
+    27 : "esc",
+    38 : "up",
+    40 : "down"
   };
 
   var ns = $.namespace('pskl.service.keyboard');
@@ -11,7 +14,7 @@
         // key is 0-9
         return (keycode - 48) + "";
       } else if (keycode >= 65 && keycode <= 90) {
-        // key is a-z, we'll use base 36 to get the string representation
+        // key is a-z, use base 36 to get the string representation
         return (keycode - 65 + 10).toString(36);
       } else {
         return specialKeys[keycode];