Mod utils

The show_map function has the "log" keyword argument now to show the log of the maps. For the steeply falling spectra that we are dealing with this might be necessary.
Also, the default label for the zenith axis is fancier now:
![index](https://cloud.githubusercontent.com/assets/7225179/3749892/601ea610-17f0-11e4-8430-08c19e76fead.png)

What happens if `vmin/vmax` are not given but `log=True`?