allow maven.build.dest to be a path instead of a single location

I'm working on some improvements on the jcoverage plugin and came accross a classpath issue. Basically, I want jcoverage to leave some classes un-instrumented. Apparently, I get ClassNotFoundExceptions when running the test-cases, because the maven.build.dest property is referenced in classpaths of the test:compile/test:test goals as a pathelement-location and it is set in the jcoverage plugin to point to the instrumented classes.
To be able to run test-cases against the instrumented and the original classes the maven.build.dest can be referenced as pathelement-path. Hence I can set the maven.build.dest property to be a path property.