Add job detail for delayed_job

Hi bugsnag-team,

I added job detail into bugsnag notification for delayed_job errors. Could you consider to merge this improvement?
The job payloads are variable and might not have necessary methods, so I add very redundant conditions to make sure the method existence.

## PerformableMethod

![screen shot 2014-06-19 at 9 45 04 pm](https://cloud.githubusercontent.com/assets/1162120/3327475/ca91f2ba-f7af-11e3-8c9b-8819b37f5b5a.png)

## PerformableMailer

![screen shot 2014-06-19 at 9 51 52 pm](https://cloud.githubusercontent.com/assets/1162120/3327516/8134c948-f7b0-11e3-9bee-fb673226469a.png)

## Your Custom Job

![screen shot 2014-06-19 at 9 45 42 pm](https://cloud.githubusercontent.com/assets/1162120/3327485/ee376876-f7af-11e3-835c-22e101acd0f6.png)