Frame content decoding refactor.

 - frame::Content values are now structs instead of strings. This allows
   multiple values and non text types to be more easily held and
   referenced.
 - Frame content decoding is now simplified with macros.
 - Picture and PictureType have been moved into the frame module along
   with the new frame content structs.
 - _enc versions of lyrics and comment setters now require a language
   string as well