Using cacheToFile() to avoid recomputing TOC

Profiled various caching approaches for getting the TOC in Gtkdoc.php:
uncached (always parsing the GtkdocFile), chached (parsing once per
request) and cachedToFile (parsing once every 3600 seconds). Checked the
results with:

ab -n100 -c 10 http://adg.entidi.com.local/

These are the results on my system:

			Time		Request per seconds
uncached		22.107		4.52
cached			7.197		13.90
cachedToFile		5.781		17.30