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
Shouldn't the theshold be symmetrical by construction?

Sent from my iPhone

> On 26 Nov 2014, at 11:35, Loïc Estève <notifications@github.com> wrote:
> 
> In nilearn/plotting/img_plotting.py:
> 
> > @@ -94,7 +94,7 @@ def _plot_img_with_bg(img, bg_img=None, cut_coords=None,
> >              data = np.ma.masked_inside(data, -threshold, threshold, copy=False)
> >          slicer.add_overlay(nibabel.Nifti1Image(data, affine),
> >                             interpolation=interpolation, colorbar=colorbar,
> > -                           **kwargs)
> > +                           threshold=threshold, **kwargs)
> Now I am wondering whether the masking inside [-threshold, threshold] l.93, 94 is necessary. The add_overlay will do the masking.
> 
> —
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