#7 code for writing tags in "hard general case"

[Vickenty Fesunov]

Currently available version 0.15.1b of libid3tag
doesn't contain code to write tags larger than one
originally found in file, or creating new tags.

Code for that part was omitted from the library for
some reason.

This patch adds the missing code.

[Vickenty Fesunov]

Patch to write tags in "hard general case".