Change order of Rack middleware execution. Fixes issues experienced with JSONP and Grape Entities

Modified execution order of Rack middleware.
Added a test for JSONP.
Added a section to the README regarding JSONP and CORS.

Fixes issues encountered with `Grape::Entity` being processed by JSONP middleware before being processed by the Formatter middleware. See #345 .

May fix other content-type header issues and body formatting related issues when using external middlewares.

Related discussions:

https://github.com/intridea/grape/issues/345
https://github.com/intridea/grape/pull/346