CloudFront invalidation batch support

[gtaylor]

Amazon CloudFront does not automatically detect file changes, even when using an S3 bucket as an origin. The solution to this is that after a syncmedia, a CloudFront invalidation batch must be formed and sent.

Here are details:
http://docs.amazonwebservices.com/AmazonCloudFront/latest/DeveloperGuide/index.html?Invalidation.html

This is very specific to CloudFront, not S3. This will probably require some thought to do in a sane way. Perhaps even a post-sync hook that the user can use on their own.

[andymccurdy]

I've posted an alternative strategy to this at https://github.com/sunlightlabs/django-mediasync/wiki/CloudFront-support

Our current in-house solution works this way. We tried CloudFront Invalidation with varied results. Sometimes it still takes time for edge nodes to invalidate our assets. Baking in version numbers to the path of the file was the only way to be sure we were requesting the new file.