Printing: print x**-1.0 differently from 1/x

`x**-1.0` and `x**-1` should print differently.  Fixed this and added
tests.  Pretty printing seemed to work already but tests added.
Issue #2860 is related and seems fixed; added tests.
On Tue, Sep 02, 2014 at 04:40:02AM -0700, Colin Macdonald wrote:
>    In sympy/printing/str.py:
> 
>  > @@ -485,7 +485,8 @@ def _print_Pow(self, expr, rational=False):
>  >                  # Note: Don't test "expr.exp == -S.Half" here, because that will
>  >                  # match -0.5, which we don't want.
>  >                  return "1/sqrt(%s)" % self._print(expr.base)
>  > -            if expr.exp == -1:
>  > +            if expr.exp is -S.One:
>  > +                # As above, don't test with "==" here.
> 
>    was the same style as line 485 just above, but I've fixed both now.

Ok.  Lets keep things as is.