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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Comienza en api.js, en el código enlazado de registerEvent() y registerEvents(), y reproduce el uso programático con varias bases de datos para observar las advertencias del listener de unhandledRejection. Define allí la limpieza mediante unregisterEvents() y verifica que el uso repetido ya no acumule listeners ni emita advertencias de fugas.

Escrito por el modelo de indexación a partir del texto del issue.