src/io/source_avcodec.h: add documentation
src/io/source_avcodec.h: add _get_duration
src/: change c header identifiers (see #35)
src/io/*.{c,h}: added const qualifiers to unmodified pointers
src/io/source*: add _close function
src/io/source_avcodec.c: add _multi, building AVAudioResampleContext the first time _do or _do_multi is run
src/io/source_avcodec.c: added first draft