Added support for :content_type and :content_md5 to AWS:S3::S3Object#url_for.

The S3 signature requries the content-md5 and content-type headers to be
signed.  The #url_for helper method previously did not accept these two
values and therefore was only good for requests where these values
were not specified.

You can now specify them like so:

    s3.buckets['bucket-name'].objects['key'].url_for(:write, {
      # both of these are optional
      :content_type => 'text/plain',
      :content_md5 => '...',
    })

Fixes #207