Add TypeScript definition

Resolve #552.

package.json

@@ -54,6 +54,7 @@
       "jquery": "npm:jquery"
     }
   },
+  "typings": "perfect-scrollbar.d.ts",
   "scripts": {
     "test": "gulp",
     "before-deploy": "gulp && gulp compress",

perfect-scrollbar.d.ts

@@ -0,0 +1,29 @@
+interface PerfectScrollbarOptions {
+  wheelSpeed?: number;
+  wheelPropagation?: boolean;
+  swipePropagation?: boolean;
+  minScrollbarLength?: number;
+  maxScrollbarLength?: number;
+  useBothWheelAxes?: boolean;
+  useKeyboard?: boolean;
+  suppressScrollX?: boolean;
+  suppressScrollY?: boolean;
+  scrollXMarginOffset?: number;
+  scrollYMarginOffset?: number;
+}
+
+interface PerfectScrollbar {
+  initialize(container: HTMLElement, options?: PerfectScrollbarOptions);
+  update(container: HTMLElement);
+  destroy(container: HTMLElement);
+}
+
+interface JQuery {
+  perfectScrollbar(options?: PerfectScrollbarOptions): JQuery;
+}
+
+declare var ps: PerfectScrollbar;
+
+declare module "perfect-scrollbar" {
+  export = ps;
+}