call CLI modules with clean environment

introducing a new module mlab_free_environment that tries to undo the
changes performed by MeVisLab on os.environ
add CLIImporter.mlab

forgot to check this in earlier.. this is needed since the introduction
of the MLABModuleHelp2Html-based screenshot generation
improved environment cleanup

also remove the MeVisLab.app/Contents/Frameworks from
DYLY_FRAMEWORKS_PATH, and remove empty variables