Improve rating and (MIME-) type handing

• #101: Determine rating based on content, as a fall-back, if MIME-type is not usable.
• #111:
  • Fix multiplicity of IRating in type definition.
  • API change: Change normalized rating range from [0..5] to [0..1]
  • Add helper function to convert rating to 5 star system
• Set common.dataformat for:
  • MPEG-4 Audio
  • Monkey's Audio
• #114: Improve MIME-type handling; allow (ignore) parameter(s)