feature : zoom

- Added MinimapController that displays a frame on the animated preview
  when zoomed in
- Added bounds for the offset to make sure it doesn't go crazy
- Added new utility Math.js with a minmax function
- TODO : the minimap controller has a lot of dependencies, see if could be
  cleaned up
- TODO : DrawingController knows the size of the picture it has to render
  only indirectly, which makes it hard in some cases (such as boundary
  checking performed during setOffset)

js/utils/Math.js

@@ -0,0 +1,9 @@
+(function () {
+  var ns = $.namespace('pskl.utils');
+
+  ns.Math = {
+    minmax : function (val, min, max) {
+      return Math.max(Math.min(val, max), min);
+    }
+  };
+})();