Add safe obj_key lookup
Remove CONTENT_CLASSES constant
Switch to postmark-rails gem
Make ContentMailer more secure; move to background job
Set FROM address to sender's e-mail
Revert "Set FROM address to sender's e-mail"

This reverts commit 6f4a090cb0345a43e5c718b47823776cae0d89ec.
Remove reference to old constant
Add some feature specs for content email
Merge pull request #187 from SCPR/content-mailer-updates

Content mailer updates