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 could have sworn I posted something about this part of the code but apparently not. There has to be a better way than doing the mapping from data space to colormap space by hand. In particular your code assumes that the mapping is linear. 

One way which may not be the best approach: im.norm is a callable that maps the data space to [0, 1] and then you can multiply by our_cmap.N - 1 to get istart and istop.