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
Thanks 

Sent from my iPhone

> On 26 Nov 2014, at 08:46, Loïc Estève <notifications@github.com> wrote:
> 
> In nilearn/plotting/slicers.py:
> 
> > @@ -14,7 +14,8 @@
> >  
> >  try:
> >      import pylab as pl
> > -    from matplotlib import transforms
> > +    from matplotlib import transforms, cm as _cm
> You can actually use pl.cm to access the matplotlib colormap module. That's what I have seen used in a few places e.g. plotting/img_plotting.py.
> 
> —
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