Use AWS CloudFront device classification if present

Add support for using the optional headers that AWS CloudFront will
populate for you based on the User-Agent. See
http://aws.amazon.com/blogs/aws/enhanced-cloudfront-customization/ and
http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/header
-caching.html#header-caching-web-device
Add tests for cloudfront
Merge pull request #25 from brycekahle/cloudfront

Use AWS CloudFront device classification if present