runtime/system/run_loop: add `invokeNext` that will invoke a function at the beginning of the next runloop
prevent animate callbacks from being called more than once
Merge pull request #806 from sproutcore/team/tim-evans/animation-callback-promises

Prevent animate callbacks from being called more than once
Added badly-needed documentation for classNameBindings.
Adds documentation for attributeBindings similarly to the addition for classNameBindings.
Tests for observers on firstObject and lastObject of SC.Enumerable.
Fix observers on firstObject and lastObject of SC.Enumerable. (#645)
Fixes up observer support of firstObject and lastObject on Enumerables.

Note: Any Object that mixes in SC.Enumerable must pass the start and length arguments to enumerableContentDidChange() for firstObject and lastObject to be independently observable.
Adds documentation on observing firstObject and lastObject.
Updated documentation for classNameBindings to fix an inaccuracy.
Removes unused variable.
Wrapped arrayContentDidChange and enumerableContentDidChange calls with begin and end propertyChange calls to prevent multiple change notifications.
Fixing spelling errors in my code comments.
Grouping property changes to prevent multiple notifications. Missed the sparse array. It was in another framework.
Fixed text field styling for IE, Opera, and mobile Safari.
Adds tests for SC.DROP_ON support for SC.ListView and SC.GridView.  These views should update the target item's isDropTarget property to reflect that it is the target of a drag.
Improves SC.DROP_ON support for SC.ListView and SC.GridView.  These views now update the target item's isDropTarget property to reflect that it is the target of a drag and drop operation, which allows us to style the drop target item appropriately.

- Previously the support was commented out in SC.GridView and was incorrectly implemented in SC.ListView (it wiped out the isSelected value on the target items).
- This fixes #197
Cleans up SC.ListView and SC.GridView drop target tests so that they can run consecutively.
Fixes syntax error in SC.DateFieldView test.
Adds unit tests for XHR2 event notification support.

These tests will fail with the current implementation.