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
> a) I would like to promote threshold=0 as a default

No. An converged algorithm (for l1 penalties) typically gives non-zero
coeffs where they are zero in practice. 1e-6 is a good compromise,
because most often values that should not be zeros are above that, and
sparse algorithms converge below that.