Enforce group invite settings

Hi Boone-

I noticed that the "send invites" tab was appearing when it shouldn't have. There were two reasons:
1) Unless the site admin visits /wp-admin/admin.php?page=invite-anyone&subpage=access-control and submits the form, the access settings aren't saved to the options table. And the options being empty causes surprising access control behavior.
2) The access control logic wasn't respecting the "invite_status" of the group.

This pull request addresses the second point. Maybe the best way to address the first is to set some option defaults on plugin activation?

The other problem I addressed here is that from the user's "send invites" tab, no access control logic was being applied (as far as I could see). Now for each group, there's check made as to whether the user should be able to send invites to that group.

Thanks!

-David