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 am not a big fan of comments that don't add much like this because they tend to become out of sync with the code. If you think it was not clear enough, why not do something like (I assumed the not so clear part was what color was used for):

```python
tick_color = 'w' if self._black_bg else 'k'
for tick in self._colorbar_ax.yaxis.get_ticklabels():
    tick.set_color(tick_color)
```