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
> Sorry if I missed the point, but for me no error should be raised if the threshold is greater than any value of the map: visualizing an empty map is a perfectly valid use case.

I kind of agree on this one. I think the grayed cmap logic has to handle this case.

> Similarly, I don't see immediately why negative thresholds are forbidden (maybe I'm missing some context here).

At the moment, the treshold logic is such that values in [-threshold, threshold] are masked so it doesn't make too much sense to have a negative threshold (full disclosure: currently it may well be the case that `threshold=-threshold` works exactly the same as `threshold=threshold` by accident)