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
This magic number happened to be the default for threshold in other parts of the code.

Sent from my iPhone

> On 26 Nov 2014, at 09:10, 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:
> Same comment as above, why is there a special case for 1e-6?
> 
> —
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