Allow Range to accept Infinites

On Sat, Jul 19, 2014 at 08:51:13AM -0700, Harsh Gupta wrote:
>    In sympy/sets/fancysets.py:
> 
>  >
>  > -        return Basic.__new__(cls, start, stop + step, step)
>  > +        step = abs(step)
>  > +        stop = stop + step
>  > +        if start is S.NegativeInfinity and stop is S.Infinity:
> 
>    Will have to use == here, is does comparison but checking the id both the
>    tuples will different have id.

Feel free to keep this as is.  That was a suggestion.