Remove modules

This pull request removes the modules from XP 5.9 again. This is based on the discussion in pull request #233. Although we agreed modules themselves are a good idea, we deemed them not ready for production. Steps taken:

* Remove module class
* Remove ClassLoader checks for module.xp
* Add VERSION file again and change xp::version() to use it
* Remove test classes
* Remove test fixtures

This will move all modularization efforts, including xp-framework/rfc#262, back to XP 5.10 (or 6.0).