Compare file contents when checking for changes

[cjonesy]

Currently we just look for an existing file, but we should be doing something like an md5 comparison.

S3 has an etag that is easily accessible without downloading the file, but it is never going to match Artifactory md5... but perhaps we could store the S3 Etag in a property in Artifactory and use that for comparison without having to download the file everytime?