Allow debug flag to be a function.

I was thinking about making some Express middleware that will allow showing the queries that are being executed and how long they take.

At this point, the best that I can come up with is making the `debug` config a callable. I don't really like this, though, and I think it'd be better to listen for some events. The following events should suffice to build this:

- `query`
- `queryComplete`

The current `query` event doesn't seem to make it's way back to the `knex` instance all the time, though. I've created another pull request, #336, to resolve issues with the events making their way back.

Would you guys be open to adding another event for when the query completes?