see #548, different way to handle multiple sampint

this should just show an alternative possibility to handle multiple
sampling rates for obspy-scan.
Merge branch 'master' into scan_multiple_samplingrates
skipping zero length traces

they'll produce a runtime division error
obspy-scan test for multipe sampling rates

bugfix comparison function + test
pep8
Merge pull request #654 from mbyt/scan_multiple_samplingrates

obspy-scan: support for multiple samplingrates