OG updated - private groups

I committed new code to OG today and rolled it to this site. We are now capable of completely private groups. Although we always have supported private content in a group, the group homepage has always been public. Thats not so anymore. A group can choose to be private now. I don't expect much use for that here, except for private planning groups like Drupalcon, Google Summer of code mentors, and so on.

Please let me know if you see funny behavior related to posting content or creating groups.