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

Conflicts:
	src/tracker-extract/Makefile.am
tracker-extract: Added png-faster-ivi