Do not write track-level genres if source is not 'track'.
For 'Various Artist' compilations without a usefull 'album' genre, pick the most used track / track-artist genre.
Try to guess 'Various Artists' genre based on track genres if source is 'track'.
Do not write track-level genres if source is not 'track'.
Fixed error message for invalid config option.
Merge pull request #96 from pscn/master

Addition to pull request #92 (lastgenre) regarding track level genres and Various Artists compilations
docs & style for #92

We already have a nice plurality() function in beets.util to calculate the most
popular value of something, so I reused that.
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