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
Rather in add_overlay then?

Sent from my iPhone

> On 26 Nov 2014, at 08:54, Loïc Estève <notifications@github.com> wrote:
> 
> In nilearn/plotting/slicers.py:
> 
> > @@ -406,7 +407,20 @@ def _map_show(self, img, type='imshow', **kwargs):
> >              ims.append(im)
> >          return ims
> >  
> > -    def _colorbar_show(self, im):
> > +    def _colorbar_show(self, im, threshold=1e-6):
> > +        # sanity-check threshold
> > +        if threshold != 1e-6:
> > +            if threshold < 0:
> > +                raise ValueError("The set threshold (%f) can't be negative"
> Not really convinced this function is the best place to do this kind of safety checks ...
> 
> —
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