Major reorganization and improved structure

- Pulled machine backgrounds and css into a "skins" directory
- Pulled emulator sources into "emulators" directory
- Artwork for UI elements are now in "ui-artwork" (leaving "artwork" for
site-specific resources)
- Added fix for PgDown and PgUp keystrokes
- Now uses HTML 5 history manipulation
- Flattened directory structure and used mod_rewrite to provide clean
Wikipedia style URLs (e.g
http://retroweb.maclab.org/articles/Apple-Macintosh)