[PRD] Bind Action added to spec

following up on ACTION-620 I agreed to split the Do action into a Do 
action and a Bind action.  Do is a simple sequence of actions.  Bind 
allows single assignment local variables.  The semantics is perhaps too 
informal for bind, but it is a start.  See
http://www.w3.org/2005/rules/wiki/PRD#Actions