Added start event to jobs

Agenda is totally sweet!

I like to see when jobs start. Mostly to preserve my sanity when I'm looking at logs and go "Did that job ever start?" - so I added a `start` event.

I also:
* Added documentation in the readme for the `start` event.
* Added tests for the `start` event (everything passes).
* Fixed typo in test name for `fail:job` event.
* I'm sorry about the whitespace removed by my editor.

Thanks for such a powerful library :beers:!