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
Please refrain to do broad PEP8 fixes like the ones you did in this commit. This is likely to cause merge conflicts for ongoing PRs. The rule of thumb I go with is only fix PEP8 in the vicinity of the lines I touched.