tracker-extract: add faster PNG extractor

This extractor reads metadata properties from the tEXt and iTXt sections of PNG
files, and skips IDAT sections (this is where the increased speed comes from)

Conflicts:
	configure.ac
	src/tracker-extract/Makefile.am
tracker-store: Show g_critical error when ontologies directory is not readable
tracker-extract: Only check ID3v1 when all ID3v2 fields are missing

Conflicts:
	src/tracker-extract/tracker-extract-mp3.c
Added README for Tracker-IVI
build-scripts: Added build-tracker-ivi script which invokes autogen.sh with correct parameters