Check for UnknownFrame when converting ID3v2 frames (#31)

Looking at the ID alone to determine the frame class is not enough. E.g.
for deprecated frames like TDAT, TagLib keeps the frame ID but returns
it as an UnknownFrame.

This change fixes the handling of such frames and adds a note to the
documentation of frame_list.
Merge pull request #32 from robinst/unknown_frame

Check for UnknownFrame when converting ID3v2 frames (#31)