This seems to be the more appropriate fix for the .log error

I previously submitted a PR for this issue. I get "Cannot call method 'emit' of undefined" and I'm not sure why this ever works, as you can see at https://github.com/pocesar/promised-jugglingdb/blob/master/lib/schema.js#L479 the log function is trying to do this.emit, but the `log = (this|schema).log` is not preserving 'this'. This seems to be the correct fix.