Revamped write functionality

I did a total rewrite of the tag writing code. Tags that weren't present in the original mp3 can now be added and their contents are no longer limited to the length of the original tag contents. I also added basic support for attaching PNG album art to files. See my commit message here for more:
https://github.com/richardadjogah/id3_reader/commit/4922ab5d2e65f5bcb01f8b65fda004a27e283b46