Use kramdown as yard markup provider

It's pure Ruby (and licensed under MIT since 1.0), so it's not a problem
to have it as a development dependency. Yard seems to have problems when
the markup provider is not an explicit bundler dependency.
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.
Update copyright
Add new file to gemspec
Add Jacob as author in gemspec
Document CHANGES for 0.6.0
Add date for 0.6.0
Add ext/taglib_flac/extconf.rb to files in gemspec
Update features in README
Bump version to 0.6.0