Refactor namespace and version setup

This PR changes the following:

* Introduce `GeoExt.Version.version` which contains the GeoExt version
* Introduce `GeoExt.Version.environment` which will also print the versions of ExtJS and OpenLayers
* Make `GeoExt.version` an alias for `GeoExt.Version.version`
* Require the base class in all relevant files

Please review.