Rails 2 support, minor logging fix, allow for multiple context_to_log fields

Unfortunately, we are still on Rails 2.3.18 (but moving to Rails 3 soon), so I had to backport the gem to Rails 2 (only the model/migration generator was affected). I couldn't get a namespace generator working so changed the generator name to StateMachineAuditTrailGenerator, which slightly changes the options to the generate command.

Aside from Rails 2, by default, events specified as symbols in the state machine model were being stored as serialized YAML strings (e.g. "--- :reset\n"), so I changed the logging to force those events to be strings before they were stored.

Also added support for multiple context_to_log fields from the state_machine model.