Deal with unexpected or missing SoundCheck values
Deal with unexpected or missing SoundCheck values

* Deals with gain values of 0 in SoundCheck files by interpreting them
  as 0.0dB rather than causing an exception
* Return default values if SoundCheck tag is malformed in some way
* Other misc changes for compactness

google-code-export/beets,google-code-export/beets,google-code-export/beets