ignore coverage dotfile
add office_slug to Office, move slugifying to helper methods
add a test for office slugs, fix agency slug test
address comments, and mistaken unique flag on office_slug
didn't mean to include this migration, but now that it's here...

may as well allow the foreign key to be null as intended
Merge pull request #197 from 18F/office-slug

Adds office_slug field to Office model