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)
Added README for Tracker-IVI
Added Tracker-IVI config
tracker-extract: Make PNG extractor optional, using --enable-libpng
tracker-extract: Make the AbiWord extractor optional, using --enable-abiword
tracker-extract: Make DVI extractor optional, using --enable-dvi
tracker-extract: Make MP3 extractor optional, using --enable-mp3
tracker-extract: Make PS extractor optional, using --enable-ps
tracker-extract: Make text extractor optional, using --enable-text
tracker-extract: Make icon extractor optional, using --enable-icon
build scripts: Disabled abiword, dvi, ps, text and icon extractors in default Tracker-IVI config
Merge branch 'png_faster'

Conflicts:
	autogen.sh
	configure.ac
	src/tracker-extract/Makefile.am