Unbind uiScrollfix directive jQuery event handlers to prevent memory leaks

The `uiScrollfix` directive will bind scroll event handlers to the window object in the absence of a parent `uiScrollfixTarget` directive. These event handlers were not removed when the directive was destroyed.

We prevent this by removing bound jQuery scroll event handlers when the scope is `$destroy`ed. Tests have been updated to verify events are correctly removed.

Resolves #119