Use Ant Contib outofdate task to avoid rebuilding artifacts unnecessarily.  Allows for proper, quick incremental build.
Don't use outofdate task for Cobertura instrumentation.  If the tests are run twice with the same Cobertura data file, the results will be aggregated, which is probably not what we want. If we delete the Cobertura data file, we have to delete the instrumented classes as well, so we do that.