members_setup/boostrapped hook

[justintadlock]

We need a hook to say that "members has been set up or is installed". This would be an early hook when the plugin is first bootstrapped to give an opportunity for add-ons to know when it's OK to run.

It's something that I've been thinking about but haven't created a ticket for. This will avoid the "white screen of death" for add-ons being activated before Members as well.