CSP 1.1: Refactor 'referrer'.

Based on the discussion at [1], this patch cleans up the naming of the
referrer directive's values, and addresses Facebook's request to be
able to send full referrer information to same-origin destinations,
while sending only the origin to cross-origin destinations.

[1]: http://lists.w3.org/Archives/Public/public-webappsec/2014Jan/0206.html