Improved visibility options

I realize that this module is unsupported and has no stable releases, but it's still a great module and I'm using it on a couple of sites.

I needed to improve the visibility options for when SPM is displayed, so I made a few code changes. I added the ability to disable SPM based on permissions and paths. I also changed the logic check for how SPM was disabled on admin pages -- instead of arg(0) == 'admin', it's better to rely on the API function path_is_admin(current_path()).

Patch is attached below.