FIX: restore np.abs ufunc support for scipy.sparse

`np.abs(csr_matrix(-np.ones(3)))` use to work in numpy < 1.9. With numpy and scipy master it triggers:

    TypeError: __numpy_ufunc__ not implemented for this type.

This PR should restore the support for the `np.abs` function on scipy sparse matrices by mapping it to the builtin `__abs__` ufunc.