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
I don't get your question. Basically all I am saying is that in img_plotting.py you masked the data inside [-threshold, threshold] [here](https://github.com/banilo/nilearn/blob/colorbar/nilearn/plotting/img_plotting.py#L94) but then you call add_overlay with a threshold argument which will mask the data again [there](https://github.com/banilo/nilearn/blob/colorbar/nilearn/plotting/slicers.py#L335).