Refactored Maven build file structure.

Used to have a bare bones aggregator at the root, and a parent POM
at the same level as the submodules. This worked fine until trying to use
the Maven Release Plugin to perform a release. The release plugin
wants to include the aggregator in the release process, which doesn't
make sense for us. Refactored to a slightly simplified approach where
the POM in the root acts as both the parent and aggregator. The same
approach is taken by projects like the OWL API.