Add the ability to group image assets into a .zip file, removing the number of indivi...

Add the ability to group image assets into a .zip file, removing the number of individual file requests.
To use, supply a Flash parameter with the location of the zips and the place they're "substituting" for, like this:
	args["assets"] = "assets/icons.zip=icons/;assets/features_pois.zip=features/pois/";
Current limitations:
- Preloader indication is not smooth. (There was a trac ticket for this anyway :) )
- Only features and MapCSS icons are loaded - other image references don't yet look in ImageBank
- Nested folders in the .zip file are not supported (a limitation of the library we're using)