Add Grape support

Since Sinatra has support, figured I'd try a hand at getting Grape to also be supported for traces.  

This seems to work, but does require putting `use Oboe::Rack` in the Grape application since it doesn't seem to like to take the middlewares when subclassing Grape::API.

I tried putting `::Grape::API.use Oboe::Rack`, and while `::Grape::API.settings.stack` knew about the middleware, they weren't inherited.