Add support for onejar-maven-plugin

The onejar-maven-plugin http://code.google.com/p/onejar-maven-plugin packages a project with all its dependencies into a single jar. The dependencies being nested jars inside the one jar.
This is a nice way to make an executable jar for a java-se standalone project.
However the current org.jboss.weld.environment.se.discovery.URLScanner does not support this as the classes are not in the expected place.

Attached is a first attempt at modifying the URLScanner to support the onejar-maven-plugin