added  back to error response bodies. Fixes #549
Add retry-after header to backoff-style error responses.
add content-langauge header to sent emails
Merge pull request #554 from zaach/content-language-header

add content-langauge header to sent emails
add accept-language to cors headers
Merge pull request #559 from zaach/cors_accept_language

add accept-language to cors headers
Merge pull request #553 from mozilla/rfk/retry-after-header

Add retry-after header to backoff-style error responses.
first steps in lastAuthAt
documented lastAuthAt