Improve Active Record instrumentation

I've implemented some monitoring of an app using AS::Notifications, but the instrumentation of Active Record is somewhat rudimentary.

This pull request adds the SQL "operation" to the event payloads (I'm not 100% sure that's the best name.) For instance, a `SELECT` query will emit an event with a payload including `:operation => :select`. Only `SELECT`, `INSERT`, `UPDATE`, and `DELETE` are supported.

Furthermore, the ExplainSubscriber has been updated to take advantage of this new payload so it doesn't have to parse the query itself.