Support saving ID3v2.3 with MPEG::File#save (#17)

The save method with the third argument (for specifying ID3v2 version)
is only available in TagLib >= 1.8, so it required special wrapping to
stay compatible with 1.7.

To make checking for the underlying TagLib version possible, expose the
TAGLIB_MAJOR_VERSION, TAGLIB_MINOR_VERSION and TAGLIB_PATCH_VERSION
defines.