fix EnablePromises method for TypeScript (#37)

Author: Vladimir Upirov

libs/backendless.d.ts

@@ -35,6 +35,12 @@ declare module __Backendless {
      **/
     function setUIState(state:string):void;
 
+    /**
+     * @public
+     * @type: Function
+     **/
+    function enablePromises():void;
+
     /**
      * @namespace Backendless.Utils
      **/

tests/tsd.ts

@@ -17,6 +17,7 @@ function testMain() {
 
     Backendless.setUIState('state');
     Backendless.setUIState(null);
+    Backendless.enablePromises();
 }
 
 function testUtils() {