added thresholded colorbar

attemps to adress #231

Here is an example for threshold=5 of what it currently looks like:

![bildschirmfoto 2014-11-25 um 18 53 21](https://cloud.githubusercontent.com/assets/3907396/5191981/85c179e6-74f3-11e4-8def-b032366a3684.png)

Greetings from Berlin
Ok

Sent from my iPhone

> On 26 Nov 2014, at 09:11, Loïc Estève <notifications@github.com> wrote:
> 
> In nilearn/plotting/slicers.py:
> 
> >          ticks = np.linspace(im.norm.vmin, im.norm.vmax, 5)
> > -        figure.colorbar(im, cax=self._colorbar_ax, ticks=ticks)
> > +        bounds = np.linspace(im.norm.vmin, im.norm.vmax, 100)
> > +        our_cmap = _cm.jet
> > +
> > +        if threshold != 1e-6:
> > +            # some colormap hacking
> > +            cmaplist = [our_cmap(i) for i in range(our_cmap.N)]
> > +            rng = float(im.norm.vmax + abs(im.norm.vmin))
> > +            start = (rng - 2 * threshold) / 2  # in data space
> > +            stop = rng - start
> > +            istart = int(our_cmap.N * start / rng)  # colormap space
> > +            istop = int(our_cmap.N * stop / rng)
> > +            for i in range(istart, istop):
> > +                cmaplist[i] = _cm.gray(i)
> I believe we want to use a single gray color here. _cm.gray is a variable gray colormap.
> 
> —
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