Made a pair of function public that are required to work with rbx.
Updated README with newer AWS Storage Gateway API version.
Added support for response targets.

You can now specify the target for a HTTP response body
when sending a request.  Example:

    # download the object to a file
    s3.get_object({bucket:'bucket-name', key:'filename'}, target: '/path/to/file')

    # download to an IO object
    buffer = StringIO.new
    s3.get_object({bucket:'bucket-name', key:'filename'}, target: buffer)