This makes Pythran numpy backend only activated when such directive is
used. Update the documentation accordingly.

When the user asked for it (thanks to the --np-pythran flag), use
Pythran's Numpy implementation as a backend for numpy operation. This
flag forces the C++ mode, as the Pythran implementation is written in
C++. Distutils integration through the 'np_pythran' flag of the
cythonize API is also provided.

This commit also adds a Pythran mode for the tests, that can enable the
pythran mode for the C++ tests, and allows the reuse of Cython tests to
test for the Pythran integration.