Remove unused locals.
Fix JobBatchList so it is properly Enumerable.

`JobBatchList#select` would iterate over the struct properties
rather than the job batches in the list. By not subclassing Struct,
it ensures that all Enumerable methods are based on our `each`
implementation.

Fixes #71.
Add `all_with_external_dependency_timeouts`.

This makes it easy to get a list of all batches that
had any dependency timeout.
These methods don’t guarantee ordering and our spec shouldn’t rely on that.
Merge pull request #77 from seomoz/all-with-timeouts

All with timeouts