Access assets using 'clojure.java.io/resource'

Instead of listing all the files in all the JARs on the classpath (with exception of `~/.m2/...` which, btw, makes testing assets in such JARs on the REPL impossible), this PR finds the desired directory (either on the filesystem or within a JAR) and creates the file list in a more targeted manner.

I hope you can use this but I might be overlooking some use case where this approach breaks things. 

Tests are passing.