Add support for WAV

- add support for
  - TagLib::RIFF::WAV::File class
  - TagLib::RIFF::WAV::Properties class

- added swig code for TagLib::RIFF::WAV classes.
- added test code for reading/writing tags (ID3v2)
    to/from WAV files, including cover-arts.
- added a WAV-encoded audio file with ID3v2 tags for tests.
- added documentation.

- Note: support for RIFF-INFO tag is not included,
    TagLib 1.7 does not support this.
Maybe we could move the set-up of this `@picture_data` variable to the 'APIC frame' context?