another try at fixing history states

I've taken another try at fixing the algorithm.  I have defined 
getEffectiveTargetStates, which dereferences history states. However, 
addDescendentStatesToEnter does not call it, since it dereferences 
history states directly and records the executable content, etc.  I've 
also changed computeEntrySet so it does _not_ place the target set of 
transitions on statesToEnter.

Take a look and let me know if you think this is correct.

http://www.w3.org/Voice/2013/scxml-irp/SCXML.htm
http://www.w3.org/Voice/2013/scxml-irp/SCXML_Diff.htm
-- 
Jim Barnett
Genesys