Make job history more first class.

It's painful to work with history timestamps as integers rather
than time objects in ruby. This fixes that.

- Rename `history` to `raw_queue_history`.
- Make a `history` alias for `raw_queue_history` that prints
  a deprecation warning.
- Add a `queue_history` method that includes parsed timestamps.
Add `Job#initially_put_at` method.
Merge pull request #88 from seomoz/deserialize-history

Deserialize history