Cloudfront detection shouldn't rely on user-agent #141

It looks like the code only detects Cloudfront if the user-agent is Amazon Cloudfront. It should instead just detect the 2-3 device headers it uses and just use them if they exist.

We've been using Cloudfront device detection for a while, and our user-agents are whatever the user's browser is, not the value above. The change requested would accommodate a larger audience and configuration setups.