Obspy scan tests

Adding test for obspy-scan. It also makes obspy-scan more robust to sampling_rate=0 and crazy station names.

@barsch: I wanted to check the coverage of our scripts, in this case obspy-scan. However, they are commented in `.coveragerc`:
```
[run]
omit =
    */scripts/*
    */data/*
    */docs/*
```
Does this have a special reason?