Rewrite the app to make it compatible with non-rails apps

This is a WIP to rewrite the app and make it usable with non-rails apps.
This idea is to have a rack middleware, which we add on top of the stack and adds us a `biceps.versions` environment parameter which we can use later on.

The rails routing constraint would then use that env parameter to route or not.
And any other rack app can use that same parameter to do it's own routing.