Cleanup pom version numbers

Currently, maven spits out a bunch of warnings when building that it can't find a property for a handful of plugins. This is a bit noisy and can be worrisome for newbies. Also, it would be good if we know exactly what versions of things are used, rather than hoping we get the right version (and that nothing has been broken in a newer version).

Also, it would be nice if all the properties lived in a central place, so its easier to keep track of certain values.