First pass at proper POMs for building and packaging things.

The gdx-tools artifact even properly bundles the various extra jars into
itself. The only thing still missing is a POM to create the gdx-platform
artifacts, which I'll tackle next.
Name the parent gdx-parent.

Everything else is gdx-something, so it's weird that parent is bare.
Maven wiring to build natives jars.

This currently downloads the jars from the build server. It may be possible to
have Maven actually do the native builds, but that seems like perhaps more
trouble than it's worth.
Allow running particle editor via Maven.

From the top-level directory:

mvn -f extensions/gdx-tools/pom.xml test -Pparticles
Merge branch 'poms' of https://github.com/samskivert/libgdx into maven