Fix around implementation

It is only possible to have a numexpr for floating point expressions.
I don't get it. If decimal is positive, nothing have to be done for integer.

Also, why the formula should change? It compute exactly the same thing no? I mean, dividing by a negative exponant and multiply by its positive value is the same.

Also, I know it is only an optimisation but look at : https://github.com/pbrunet/pythran/commit/20be15dfc7242cd696293f795c346dd59122fe3c
I think you can do the same for this case.