Make sending alerts on job failures optional. Disabled by default.

I setup Rearview for a couple of live services. It works great except it sends out Email and PagerDuty alerts even when the job fails for "internal" errors. For example, in cases where there was a one-off error fetching data from Graphite or when the monitor timed out. Such cases don't warrant waking up someone in the middle of the night!

Please see [issue 8](https://github.com/livingsocial/rearview/issues/8) for a couple of examples.