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.
adapt existing tests to last commit and add test for java.net.URL instance
Merge pull request #50 from schmir/jar-file-resource-path

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