This save custom data in user_dir and make a content.txt (a md5sum
	like file) listing custom files. Simply put this files in a web server
	to diffuse in classroom.

	GCompris clients are launch with --server and --cache-dir
	On start, client download content.txt from server.

	And when a client need a file:
	- first he search in server list (from content.txt)
	- if found, he check md5 of file in cache_dir
	- if md5 isn't ok, file is download in cache_dir
	  file in cache_dir is used
	- if not found, search in package_dir (like now)

svn path=/trunk/; revision=3720