src/ima_adpcm.c : fix for IMA4 AIFF-C, closes #2

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
src/ima_adpcm.c : Add parens for explicit 16 bit sign extention.
src/ima_adpcm.c : 2 separate IMA seek functions

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>