TradableEvent -> TradableEventInterface, TestTradableEvent

Some refactoring to prepare for the creation of a TradableEvent model class
in the TAT system: treve was moved/renamed to test/test_tradable_event.rb
(TestTradableEvent) and a new (module) parent, TradableEventInterface was
created - included by (test/)TestTradableEvent and to be included by the TAT
TradableEvent model class.  Tests using TradableEvent were adjusted to fit
these changes - tests pass.