Properly handle testing of external modules.

To properly handle testing for external modules, four changes were needed:

1. MODULE_DIR/spec has to be added to rspec's include path.
2. MODULE_DIR/lib has to be added to rspec's include path.
3. A helper script is used to properly set the module and manifest
   directories.
4. MODULE_DIR/lib is added to $LOAD_PATH by the helper script to make
   sure that custom fact definitions are loaded.