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
Can you use `threshold=threshold` i.e. use a keyword argument here? I reckon this is good practice, see for [example](https://code.google.com/p/soc/wiki/PythonStyleGuide#Function_Arguments)