Negative step when slicing

[John Schulman]

Currently you can't do x[::-1] or x[:,::-1]. Currently x[start:stop:step] produces a GetSli Op with those arguments, however some of the logic assumes that step is positive. We have two choices

1. generalize GetSli to work with negative step
2. assume that if a symbolic value for step is provided, it's positive. If it's negative at runtime, throw an exception. If it's non-symbolic and negative, use a Flip Op.

[John Schulman]

Fixed in a43259d
step must be an int when you call the indexing functions.
Hopefully no one needs step to be determined at runtime.