EventEmitter memory leaks

When using `db-migrate` programmatically for multiple databases, warnings are generated for EventEmitter leaks: `Warning: Possible EventEmitter memory leak detected. 11 unhandledRejection listeners added.`

Adding a `unregisterEvents` function to `api.js` should enable programmatic users to get around this. Such a function may also be used more generally to do proper cleanup.

Where `registerEvent()` is called:
https://github.com/db-migrate/node-db-migrate/blob/master/api.js#L136-L147

Where `registerEvents()` is defined:
https://github.com/db-migrate/node-db-migrate/blob/master/api.js#L136-L147
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Start in api.js at the linked registerEvent() and registerEvents() code, then reproduce programmatic use across multiple databases to observe the unhandledRejection listener warnings. Define unregisterEvents() cleanup there and verify that repeated use no longer accumulates listeners or emits leak warnings.

Written by the indexing model from the issue text.