Updated plugin to support CDNs

I've added some app settings and change the code in the helper to support the use of CDNs with sfCombinePlugin

In app

under each section (JS & CSS)

cdn:
        enabled: true     # true if using a CDN or false if not
        host: cdn.example.com #the CDN host address
        protocol: //  #http:// for straight http://, https:// for straight https or // to determined by page

Working on ProdPad using CloudFront as the CDN.