Use dynapath for manipulating the effective classpath

This removes the URLClasspath protocol, and delegates that responsibility
to dynapath.

This change breaks backwards compatibility for any project that
used/implemented URLClasspath.
Update to dynapath 0.2.0 api.
Add clojars as a repo to pull in dynapath.
Switch to central version of dynapath.
Merge pull request #43 from tobias/dynapath

Use dynapath for manipulating the effective classpath