Fix ROF code.

The Chambolle paper this is based on --
http://www.cmap.polytechnique.fr/preprint/repository/578.pdf --
initializes Xi_0 with 0, not with g (see page 13 at the bottom).

Consider this code:

    im = misc.lena()
    U, T = rof.denoise(im, im, tv_weight=30)
    imshow(U)

Before this change, this produced the output
http://i.imgur.com/u68Vakj.jpg . With this change, it produces
http://i.imgur.com/3eHli7J.jpg which looks more correct.