Add Docker Distribution API Version header
Add Docker Distribution API Version header

Setting a header for all responses can help clients better determine
if the server speaks the legacy v1 API or the v2 API. It is important
that the header be set *BEFORE* routing the request.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

thaJeztah/distribution,thaJeztah/distribution