Use dynapath for manipulating the effective classpath

As we discussed on #leiningen, I've pulled the URLClasspath protocol out to a separate project and extended it to provide checks for readability as well (see https://github.com/tobias/dynapath).

This PR removes URLClasspath from pomegranate, and delegates that responsibility to dynapath.

This change breaks backwards compatibility for any project that used/implemented URLClasspath - let me know if you see that being a problem.