add X-Robots-Tag header

adding the X-Robots-Tag header might be a good thing, 
as it prevents the sitemap itself from getting indexed by google and showing up in the SERPs

see https://developers.google.com/webmasters/control-crawl-index/docs/robots_meta_tag
Merge pull request #74 from gherkins/patch-1

add X-Robots-Tag header