Adds stats object to foia hub model. refs #184

- Add stats obj to foia hub to store stats.
- Since, office-slug #197 was on its way to being merged, this is being submitted with that already accounted for. Migrations in this branch build off of migrations in that branch. 

(Someone let me know which way they want this pull request submitted. I will update appropriately.)


First of all that is a very good catch. I think solving might take a team effort on what was intended/what happened. This seems very weird. 

- I can't find it in the models in this branch -- https://github.com/18F/foia-hub/blob/add_report_to_db/foia_hub/models.py#L175-L189. 

- It is not in the models in the office-slug branch -- https://github.com/18F/foia-hub/blob/office-slug/foia_hub/models.py#L142-L156

- Migration 0013 seems to add field to the db.

Was 0013 manually created or did somehow it get dropped from the models, which is causing 0014 to remove it?