Fix offset saturation bug in mouse wheel handler.
Document CollectionChangeType enum members.
Clear CSS class name property.

Old code set it to the string "null".
Replace paired remove/insert events with single replace. Closes #4.

Reuse TR/TD elements during scrolling and reordering via a new
'replace' event type.
Changes occurring outside the window update the scroll bar. Closes #5.
Assign a class to the 'scroll' tr element.