fix(datepicker): correctly manage focus without jQuery present

The element.focus() will throw an error since the object needs to be unwrapped first.
Should be:  element[0].focus() at a minimum to unwrap the jqlite object since it doesn't expose focus().

Closes #758

src/datepicker/datepicker.js

@@ -391,7 +391,7 @@ function ($compile, $parse, $document, $position, dateFilter, datepickerPopupCon
           updatePosition();
           $document.bind('click', documentClickBind);
           element.unbind('focus', elementFocusBind);
-          element.focus();
+          element[0].focus();
         } else {
           $document.unbind('click', documentClickBind);
           element.bind('focus', elementFocusBind);
@@ -427,4 +427,4 @@ function ($compile, $parse, $document, $position, dateFilter, datepickerPopupCon
       });
     }
   };
-}]);
+}]);