Fix synchronous CORS requests in Firefox 37
Fix synchronous CORS requests in Firefox 37

- Extract Carnival.openRequest
- Put withCredentials = true within a beforeSend function[1]
- Resolves #248

1: http://stackoverflow.com/questions/16668386/cors-synchronous-requests-not-working-in-firefox.

peterwang/carnival,vaporware/carnival,thoughtbot/carnival