Add support for ID3v2.4 TDRC frame

[Uninen]

Adds support for TDRC frame mapping for year tag (used for example by FFmpeg and because of that cornucopia of other tools).

Didn't add a separate file for testing it because the existing tests already had handful of files that had this field set and did not report year before but now do, hence fixed the test fixtures as well. (Maybe this should be considered as a breaking change because of this?)

Also fixed a minor typo in the readme.

Fixes #155