Turn pow2 into numpy.square

Use existing numpy.square functions instead of the fake pow2 one.
Improve the implementation of numpy.square using nt2::sqr.

This fixes #292.
On Mon, Aug 18, 2014 at 02:04:38PM -0700, pbrunet wrote:
> In pythran/optimizations/square.py:
> >      '''
> >
> > +    def visit_Module(self, node):
> > +        self.need_import = False
> > +        self.generic_visit(node)
> > +        if self.need_import:
> > +            importIt = ast.Import(names=[ast.alias(name='numpy',
> asname=None)])
> I though it should be import numpy as pythonic::numpy. It doesn't matter?

you got a point...
I need to double check this.
> Double import neither?
import are not used for #include generation so it's ok.