Cache maven-ant-tasks.jar locally

After an ant realclean the maven-ant-tasks.jar file is gone.
build.xml already has some mechanism to use that file from a local maven repo - but there's no way to actually add that jar to the repo if it is not there.

Attached patch caches the jar in the local maven repo.

(Although it's not "polite" to blindly write into a user's local repo, I think it's ok to do it)

Any opinions?