Fixing UTF-8 Conversion issues in getString

This pull request will add a method for correctly converting an ArrayBuffer into a UTF-8 String.

Since __UTF-8__ can encode characters into _multiple bytes_ simply using `String.fromCharCode.apply` on single bytes will fail on multibyte chars.

I merged a working conversion function into DataView to have getString use it. Actually one could make a tiny repo for just this conversion function but then again…

Please test `id3js` against these changes. On `mp4js` I had to make sure that the frame names will __not__ be read via DataView.getString, since they are __not__ valid UTF-8 (The copyright sign in mp4 atom names is encoded as `\251` whereas in UTF-8 it is not).

Kindly,

Lennart