Util.mousePosition - use a proper positioning root

This partially reverts d45a366 by
restoring the use of the jQuery `.offset()` method instead of
`.position()` but fixes the function for the case where the passed
in element (usually the .annotator-wrapper div) is not a positioning
root. In this case, its offset parent is taken as the offset element.

Fix #243
Fix #244

src/util.coffee

@@ -159,7 +159,10 @@ Util.maxZIndex = ($elements) ->
   Math.max.apply(Math, all)
 
 Util.mousePosition = (e, offsetEl) ->
-  offset = $(offsetEl).position()
+  # If the offset element is not a positioning root use its offset parent
+  unless $(offsetEl).css('position') in ['absolute', 'fixed', 'relative']
+    offsetEl = $(offsetEl).offsetParent()[0]
+  offset = $(offsetEl).offset()
   {
     top:  e.pageY - offset.top,
     left: e.pageX - offset.left