Added Cache Control metadata to S3 upload

Added two new properties for the S3 Upload plugin, allowing to
set the default S3 cache control header, and to set for media
types (jpg, png, webm).

These properties are then used to generate the max-age property
when uploading to S3, or when checking against existing files
Merge pull request #125 from jrstarke/s3_cache_control

Added Cache Control metadata to S3 upload