provide ability to enable REMOTE_USER authentication within django/graphite + custom HTTP header auth. useful for external authentication capabilities

Having this patch allows me to use this graphite module, but also have a separate openid application ( http://antoineroygobeil.com/blog/2014/2/6/nginx-ruby-auth/ ), as well as using jfryman's nginx module instead of the builtin nginx support.

has zero user impact otherwise, just opens up sufficient params for custom stuff.