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
OK, but what should we do about the following use case (people do on SPM all the time): visualize dissymetric ranges of values: [th, +\infty]. This is because in many (most) neuroimaging studies, people are interested in A>B but do not care about B>A, and so do not want to have their image cluttered with negative peaks if they don't care about it.  I agree that this should not be the default, but I would leave this possibility open (through another kwarg ?).