Requested by areinwrath to support something like a conkeror/site-js for local system scripts. Seems like
a good idea to me.

It is easy enough to make application.module_uri_prefix a list of prefixes, but now we need module name
conflict resolution. It would probably be useful to use emacs' behavior as a starting point.

The current practice of tracking modules purely by module name (rather than by full module path) will
probably need to change.

When I introduced the module loading system, I expected that something like the Emacs load-path would be
implemented eventually.  The one thing that I haven't looked into is how to easily check if a chrome URL
is present.  Maybe the solution is just to try loading from each successive path, and look at the
exception generated to see if it is due to the file not being present.

Fixed.  See the new user variable load_paths.