(WIP) Added new distribution module which builds different distributable zips

This pull request is a work-in-progress and needs more discussion.

I created a new module which has the sole purpose of creating zip archives that can be offered on the Ardor3D website for users which don't use Maven etc. The zips include all necessary transitive dependencies.

It has to be done in a separate new module (which I called ardor3d-distribution) and not in the parent pom because it has to be the last build step after all other modules (including the parent project) have been build. See [the docs](http://maven.apache.org/plugins/maven-assembly-plugin/examples/multimodule/module-binary-inclusion-simple.html) for more details.

At the moment, I created assembly xmls for two zips, one for people using jogl, one for lwjgl. For the lwjgl zip I include the extracted native libs in a separate folder inside the zip. I have a problem excluding the corresponding native jars though, I'm [investigating](http://jira.codehaus.org/browse/MASSEMBLY-644?focusedCommentId=319657)...

TODO
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1. Which other zip combinations except jogl/lwjgl are useful?
2. Should the jars in the zips include sources?
3. How can we automatically offer the zips for SNAPSHOT builds?

For 1., we have to also think about platforms, e.g. different zips for each platform, although I don't see a big advantage there (in terms of (download) size).

For 2. maybe it's good to offer zips with and without sources, for development and distribution, respectively.