Allow multiple CDN urls for parallel loading

[markguinn]

You should be able to give an array of CDN urls and have it pick one randomly or round-robin each time Link() is called. This would allow you to set up multiple CNAME records to the CDN and utilise parallel connections better.

Something like:

    BaseURL:
      - http://media1.mysite.com
      - http://media2.mysite.com
    SecureURL:
      - https://media3.mysite.com
      - https://media4.mysite.com