Fixing waveform_id parameter in obspy.core.event.FocalMechanism
to list of WaveformStreamID as specified in QuakeML docs (fixes #633)

no deprecation handling as I can't find any QuakeML example containing
this parameter - for now I assume this has not been used yet - if someone
needs a deprecation handling feel free to submit a pull request

also small fix for obspy.pde - @claudiodsf please review