History event, clarification

[John Fuller <jfuller@wernervas.com>]

The idea is that when I serialize a type I include that type as an 
attribute of the element for clarity.
In the current schema, the complex type inside of history representing 
an event element isn't defined with a type name of its own.
It would be more convenient for me if AS:HistoryEvent or AS:Event were 
defined in the schema on their own
and then the Event element be of this type.
It also would let us reuse this type if we needed.