Add interface with gtags via `ggtags.el'

I recently discovered the Emacs `ggtags` plugin after getting increasingly frustrated with regular ctags/gtags. It's great, and better, it's maintained in GNU ELPA (whereas the `gtags`plugin seems older, and is only in Marmalade).

This is a very simple patch to integrate drupal-mode with `ggtags.el` for its symbol completion.  However, this may not be the best way to do it. It would probably be better to merge `drupal/gtags.el` and `drupal/ggtags.el` together, since the new file is almost an exact copy of the old one with minor changes, and the dependencies are not complex. Let me know if you'd prefer that and I can make a different patch.