Refactor Person.first_or_create

Ruby 1.9.3 destroyed our little end-around and broke this method. This commit fixes that.

The intent here is to enforce this: code should not search for people without including an email and an org. Searching without an email will get you the first person without an email. Searching without an org will get you someone in another org.

I will eventually override first_or_initialize. I will also refactor out the other two Person methods for finding people.