clean up code. compiler-warnings.
move PRi64 to local include
doxygen API doc
get rid of 'external' debug_level
fixup missing PRi64 types (commit e930f63)
prefix dump_nfo() with ad_
mark all internal functions as static
remove explicit libflac support (libsndfile and libav can read it, too)
LGPL boilerplate for audio-decoder
prepare v0.3.0 release
update copyright info
finalize changelog (v0.3.0)