add ISRC support

This prints found ISRCs on stderr, since only the actual disc ID is
given on stdout.
copy isrc rather than using (unreferenced) internal
Merge branch 'master' into isrcs

This merges the DEBUG flag and outputs relevant information on stdout,
rather than stderr.

Conflicts:
	image_id.c
Merge branch 'isrcs' of git://github.com/JonnyJD/image-id

This adds support for reading ISRCs from images and printing them as output.
Closes #1.
Drop unnecessary cast, use MIRAGE_ISRC_SIZE instead of magic numbers.