Correct handling of asset URIs
URI_scheme.named_uri is a little dodgy: it uses the absence of a file
extension as a heuristic to decide whether the URI should have a slash
put at the end. This is the "right" behaviour for tree resources, but
not for assets.
Although it would be good to clean this up more thoroughly, I have for
now introduced a new file_uri function that is used for assets.
References ~jonsterling/forester#193