TheGamesDB.net Scraper

This PR adds game support to the list of multimedia classes supported by Heimdall.

TheGamesDB.net is used as a search collector and predicate filler-inner, much like the existing tmdb scraper. A second commit enhances game_item.py, and allows it to scrape metadata directly from game image headers. I added initial support and test cases for gameboy, gba and n64 (test cases are between 64 and 300 bytes).

I'd like to pull @maloep into the discussion on this one as well, to make sure that I finished up the work he started ok and that the work in the pr is easily extensible to future scrapers and game tasks.