Fix #249: Guard against events not existing in MySQL 5.0

The only major schema component that's been added since MySQL 5.0 is events– in order to support MySQL 5.0 we need to guard against trying to load them in that version.

Note: This does not provide a MySQL 5.0 emitter mode that puts MySQL 5.0 events into the MODYLLIC table or some such, at this time.  The fact that trying to load a schema with events into MySQL 5.0 produces an error is something I consider a feature. (Long term, it will need to support this sort of thing, as cross-loading MySQL/PostgreSQL will encounter this all the time.)