make it possible to set the custom-resource-path to any URL

Currently, when the custom-resource-path is set, it always refers the
local filesystem. I'd like to be able to load templates from a jar file
without always specifying the full path. This patch makes it possible to
call set-resource-path! with a jar URL, like in

(set-resource-path! (clojure.java.io/resource "META-INF/foo/templates"))

This currently breaks the related tests and is also missing some tests
for the new functionality.

I'm willing to provide some more tests and fix the existing tests if this can go in.
Also, the 'file:///' with three slashes looks suspicious to me. Wouldn't two slashes do?