incomplete stuff
Added Stack command

stack arbitrary objects such as
"stack all red bricks"
"stack up all black objects"
"build a tower of all objects"

added stack command to the grammar
added StackNode to the parse tree model
added stack logic in Interpreter
stack is implemented as a move Action, no chages made beyond the
interpretation step.
somewhat naive implementation for now
does not generate the fastest possible solution.  Generates one
possible stack of the O(n!) different possible permutations.
does not produce special stacks with multiple tables and boxes