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
then it needs to scale with the dpi of the figure i guess. maybe overkill

On Friday, November 28, 2014, Danilo Bzdok <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)
>
> That's because I changed back to the lower level ColorbarBase() call since
> it permits a user-defined cmap as an argument. Without specifying bounds
> the colorbar did not work out.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/nilearn/nilearn/pull/296/files#r21043629>.
>