Eta times

To test this branch you will need to start data ingestion and let it run for awhile. Then run the calculate_eta command to populate the ``job_eta`` table:

```python manage.py calculate_eta```

Once populated, the ``job.pending_eta`` and ``job.running_eta`` columns will be populated as new data comes in. The ``calculate-eta`` task is set to run every 6 hours but I'm guessing no one wants to wait that long... Also, this branch will require changes in both ``treeherder-client`` and ``treeherder-ui`` to display the eta data and provide support for the new data types we ingest.

New stuff:
* job_eta table for storing pending and running eta estimates
* reference data signature, pending_eta and running_eta columns in the job table
* reference_data_signatures table in the treeherder database
* new reference data type for ``device``
* fixed the failing cycle_data test