Improved spacing in traceplot function

* use `plt.tight_layout` to automatically fix spacing
* vertical figure size based on number of variables

Example on my system:

![before](https://f.cloud.github.com/assets/383090/1410555/3c7ea0d6-3da6-11e3-89a4-035ce5785419.png)
![after](https://f.cloud.github.com/assets/383090/1410554/3c7e7930-3da6-11e3-96c4-2311524fdc04.png)