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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