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: Do not attempt to process NULL ID3 tag

When calling g_convert with a missing codepage, it returns NULL, while not
setting an error (specifically, if the CP1252 code page is missing, a
segmentation fault occurs). This means id3v24_text_to_utf8 and
id3v2_text_to_utf8 may return NULL. If NULL is returned by these functions, do
not process this tag further.
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