Added API for properties.

I seems to work but it only returns high-level list and not full list of id3v2 frames, as documented there:
  http://taglib.github.com/api/classTagLib_1_1ID3v2_1_1Tag.html#ad079c917f545a5af07db721f5cf2842a

This might need an explicit type-case, which would be a pain in the butt.