Automatic dependancy retrieval

Autopackage is capable of downloading packages from the network in order to satisfy dependencies. This functionality is provided by luau, and for it to work correctly a few pieces of information are required:

The URL is stored in the Repository key of the skeleton file. This isn't the only way register a repository for a particular package - you can use the registerRepository function as well. The XML file pointed to by the URL should be on an HTTP or FTP server.

FIXME: document the format here