Track metrics for failure

I think tracking failures is pretty important. It's definitely one of the more actionable metrics (*oh snap, failures increased over the past hour, better look into it*).

This adds a `on_job_failure` hook to `Resque::Metrics`. I tried to remain in line with the other hooks as much as possible, but the main difference was I didn't see an obvious way to track time since exception, and also I don't think it'd be as useful even if I included it.

I've updated the README with details, and included additional tests to confirm functionality. I ended up including https://github.com/quirkey/resque-metrics/pull/5 in this so I could get tests working locally.