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
Ups...

Sent from my iPhone

> On 26 Nov 2014, at 11:33, Loïc Estève <notifications@github.com> wrote:
> 
> In nilearn/plotting/img_plotting.py:
> 
> > @@ -86,7 +86,7 @@ def _plot_img_with_bg(img, bg_img=None, cut_coords=None,
> >  
> >      if bg_img is not None:
> >          slicer.add_overlay(bg_img,
> > -                           vmin=bg_vmin, vmax=bg_vmax,
> > +                           vmin=bg_vmin, vmax=bg_vmax, threshold=threshold,
> I don't think we should threshold the anat
> 
> —
> Reply to this email directly or view it on GitHub.
>