'Invite only' option for 'User registration settings'

This patch adds a 'New user registration by invitation only.' option into the 'User registration settings'.
When this option is selected, the 'user/registration' form cannot be accessed by anonymous users and new users cannot register without a valid invite (The admin can ofcourse still create new users).
The 'User login' block also has the 'Create new account' link removed when the option is selected.

I've not set this issue's status to 'patch' as this probably classifies as a quick hack (thus meeting the don't set to patch criteria from some doc somewhere).