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.
I quickly implemented this for my own needs, and didn't originally intend to commit it, but then I thought I may as well share it. I won't have time to create a C++ version anytime soon, as I have higher priorities right now. I can remove this ruby file if you prefer.