Interact sliders should be able to have negative step size

[kcrisman]

Stepping backwards on sliders. Currently,

@interact 
def _(n=slider(100,0,-1,1)): 
    print n 

gives the error

ValueError, "invalid negative step size -- step size must be 
positive" 

as far as I know. Allowing this could be very helpful, but maybe there was a reason not to do this I am unaware of - the explicit ValueError? leads me to think so, since whoever wrote it put knew this could happen!