[Tracker] Bug 509601: make tracker-extractor modular

[Carlos Garnacho <carlos imendio com>]

Hi!,

In bug #509601 there's a patch attached to make extractors GModules, so
tracker-extractor loads them at startup instead of having a fixed array,
This may be interesting if some application wants to provide its own
metadata extractor. I'd appreciate any testing :)

Thanks!
  Carlos