Added STBTT_PLATFORM_ID_UNICODE

Added a STBTT_PLATFORM_ID_UNICODE clause to support iOS/Mac fonts
stb_vorbis: use const char* when appropriate
stb_image: fix undefined preprocessor definition
stb_vorbis: fix signed/unsigned comparison
stb_image: fix unused parameter
stb_vorbis: fix unused functions
vorbis warnings from ronny chevalier

I manually made these changes because they
disabled some substantial amounts of code,
and I wanted to make sure this wasn't due to
original code having bugs or deviating from
the specification
stb_vorbis: fix unused variables

Conflicts:
	stb_vorbis.c
Merge branch 'rc/const' of https://github.com/ronnychevalier/stb into working
Merge branch 'master' of https://github.com/HouQiming/stb into working
comments/ version update for platformID = Unicode
update version number
version number
version numbers