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.)


I added 0013 in the `office-slugs` branch, which was a mistake -- I intended to wait for my next branch/PR to add that field to the DB. That's why the migration is there, but the models file doesn't have the field referenced. My next PR, #204, is what 0013 was meant for, and includes that field.

I forgot that Django lets you generate migrations based on a DB diff, which is I assume what you did. I've been manually creating all my migrations.

This can be solved by manually removing this instruction from the migration. I'll update the branch to do this.