5/19/14 SlopeEquationNode: mutate the interactive equation instead of using removeChild/addChild, to improve performance
5/19/14 make NumberBackgroundNode and UndefinedSlopeIndicator mutable, to improve performance
cleanup and doc
remove _.omit experiment in NumberBackgroundNode
doc
PointSlopeEquationNode: replace options.interactiveX1 and options.interactiveY1 with options.interactivePoint