Add 'element' and 'tether' to Drop typings

drop/drop-tests.ts

@@ -22,6 +22,8 @@ d.remove();
 d.toggle();
 d.position();
 d.destroy();
+d.element.appendChild(document.createElement("div"));
+d.tether.position();
 
 d.on("open", () => null);
 d.on("close", () => null);

drop/drop.d.ts

@@ -30,6 +30,8 @@ declare module drop {
 
     interface Drop {
         content: HTMLElement;
+        element: HTMLElement;
+        tether: tether.Tether;
         open(): void;
         close(): void;
         remove(): void;