Added set_once call to the library.

Added set_once to the library, as documented here
https://mixpanel.com/docs/people-analytics/javascript

mixpanel.people.set_once
This method allows you to set a user attribute, only if it is not currently set. It can be called multiple times safely, so is perfect for storing things like the first date you saw a user, or the referrer that brought them to your website for the first time. 

Also added a test case for people.set_once