Add JSON response support

Turnout is a great gem that a friend of mine found a couple of weeks back when I needed to do maintenance mode on a couple of RoR sites. I noticed that it didn't have JSON response capabilities for maintenance mode on sites like one I have that is an RoR API and shouldn't ever return HTML. So I thought, why not just add it to this awesome little gem?

I tried to stay true to what I saw as your form and to be as unobtrusive as possible. I won't be offended if you feel some of the code or calls should change, but I felt that maybe what I needed for my own project might be useful to others.

-Tristan