numexpr: new package, version 2.4

Description from the numexpr project page [1]:
> Numexpr is a fast numerical expression evaluator for NumPy. With it,
> expressions that operate on arrays (like "3*a+4*b") are accelerated and use
> less memory than doing the same calculation in Python.

I am adding it, because it is a dependency of the pytables package.

[1]: https://github.com/pydata/numexpr