Added asynch Lyrics Tag

hey,

I used your library for my mp3 tool (https://github.com/cf86/MP3ToolKit). Really nice library btw :) Therefore I added asynch lyrics tag and added the ID3v24 year tag. A Test for the changes is included too. A detailed description what I changed can is here:

AbstractId3v2Tag class:
- added asynch Lyrics ID at line 32
- added getter for asynch Lyrics at line 662
- added setter for asynch Lyrics at line 668

- extractTextFrameData at line 802 changed to protected
- invalidateDataLength at line 295 changed to protected

ID3v24Tag class:
- added year ID (TDRC) at line 7
- getter for Year at line 56
- setter for Year at line 64

ID3v2 class:
- added asynch lyrics getter and setter at line 53 und 54

ID3v2TagTest class:
- added a test for new id3v24 year and asynch lyrics at line 417

- added the file ID3v24YearWithLyrics.mp3