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 know (see my comment about promoting it to a module constant above) but still. I don't really understand why 1e-6 should be treated differently from any other threshold. Shouldn't you try to gray the area between [-1e-6, 1e-6]? It will probably only show up a a gray line at 0 I would have thought.