Add magic for Musepack SV8
According to http://trac.musepack.net/musepack/wiki/SV8Specification, the file starts with a 'MPCK' magic.
Currently, only the SV7 magic, 'MP+' is recognized: http://trac.musepack.net/musepack/wiki/SV7Specification.
Fixes #104 (closed).
Edited by StefanBruens