I have verified that the etag field is indeed returned when getting the files from the bucket:

```ruby
irb#1(#<Middleman::Application:0x70104257348120>):004:0> Middleman::S3Sync.bucket.files.last
=>   <Fog::Storage::AWS::File
    key="you-know-it-s-spring-/index.html",
    cache_control=nil,
    content_disposition=nil,
    content_encoding=nil,
    content_length=2620,
    content_md5=nil,
    content_type=nil,
    etag="f01b3a093e1774f3f9c9bd8c96f67261",
    expires=nil,
    last_modified=2013-11-20 05:31:47 UTC,
    metadata={},
    owner={:display_name=>"freddyjean", :id=>"ac8afd8c98477086b192e47017d7ada02fd2393c6faafffa61cd71481c5523f4"},
    storage_class="STANDARD",
    encryption=nil,
    version=nil
  >
```

However, I had to use a custom metadata field when I implemented gzip support. The act of recreating the file caused some internal metadata to change, changing the etag value. This is the main reason I am making a head request. The result from the head request includes the custom metadata that I attached to the request:

```ruby
irb#1(#<Middleman::Application:0x70104257348120>):005:0> Middleman::S3Sync.bucket.files.head("you-know-it-s-spring-/index.html")
=>   <Fog::Storage::AWS::File
    key="you-know-it-s-spring-/index.html",
    cache_control=nil,
    content_disposition=nil,
    content_encoding="gzip",
    content_length=2620,
    content_md5=nil,
    content_type="text/html",
    etag="f01b3a093e1774f3f9c9bd8c96f67261",
    expires=nil,
    last_modified=2013-11-19 22:31:47 -0700,
    metadata={"x-amz-id-2"=>"mJeyWTVymcGGa2PIX2oRrQgX2v5HOamYAWGvrUnoMhdcOb9vmAXIrPGR8EpQ0bM5", "x-amz-request-id"=>"8A34F666E8408814", "x-amz-meta-content-md5"=>"3ac3be7d8eae898cd584d1a02fcee6c7"},
    owner=nil,
    storage_class=nil,
    encryption=nil,
    version=nil
  >
```

I'm open to suggestions on how to reduce the number of requests made by s3_sync. I think it would help a lot. I can't however merge that particular pull request at this point though.