Adds support for SoundCheck

 * Adds 2 functions to convert between SoundCheck and ReplayGain
 * Adds relevant StorageStyle to rg_track_* for SoundCheck
 * Adds a new packing type for SoundCheck
 * Modifies StorageStyle to accept a lang argument
 * Modifies StorageStyle to allow specifying Packed out_type
 * Modifies MediaField to write COMM frames with a lang attribute
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
Merge pull request #97 from daveisadork/soundcheck

Adds support for SoundCheck