Refactor Event#associate_with_venue

* Extracted the method `Event#associate_with_venue` from `Event` and moved it to `Venue`
* Added tests for new method `Venue.find_by_identifier`
* Added myself to the contributors


A new, un-persisted venue? If so, testing that the returned value is "just" an instance of `Venue` doesn't quite cover what's going on in the implementation.