Attribute - optional opts argument of set method does not surface as an argument of the user defined setter.

Added a fix in AttributeObservable for [#2533172](http://yuilibrary.com/projects/yui3/ticket/2533172).

The problem is that the options are merged into the event
facade and cannot be extricated later on from it to
pass it on to the setter as a separate object.

My solution would be to store it as a separate property
in the same facade so that it is easy to extract.
The added property is prefixed with an underscore to
signal it as for internal use only.

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