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


An instance of `Venue` or the `venue` itself? Looking at the implementation, I think this description should read `returns the argument when passed a Venue`. Something along those lines.