Properly escape URLs added to the classpath.

The function `.toURL` in java.File has been deprecated since 1.4 as it does not properly escape special characters. (More info in the [Oracle Bug Database](http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6179468)). The recommended way to handle special characters is by converting the String to a URI first, then to a URL.

This pull request will fix the problems presented in technomancy/leiningen#1447.