fix (uiScrollfix): Unbind 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,
causing a memory leak.

We prevent this by removing bound jQuery scroll event handlers when the
scope is $destroyed. Unit tests have been updated to verify these events
are correctly removed.

Resolves #119