adds multi-tenant support

 - uses https://github.com/ErwinM/acts_as_tenant
 - tenant is based on School
 - tenant is grabbed from subdomain, so for testing, I added the following line to /etc/hosts:

        127.0.0.1       railsschool.dev sf.railsschool.dev cville.railsschool.dev

 - base domain of railsschool.org should have the same functionality it currently has after deploying (although the image won't say San Francisco in it), but I recommend just having it redirect to sf.railsschool.org or replacing with a homepage that asks the user to select which school they want to attend

 - I created a San Francisco school in the production database and linked the current venues to that school in preparation for deploy (and since it can't hurt right now anyway since these models+fields already exist)