Make CSRF failure logging optional/configurable.

Added the log_warning_on_csrf_failure option to ActionController::RequestForgeryProtection
which is on by default.

My reasoning being that I'm using papertrailapp on an app that is maybe 80% API and I'm explicitly using null_session to ignore CSRF problems on my API endpoints safely, but am getting a lot of log noise which isn't very helpful. I thought about overriding `verify_authenticity_token` in my app code, but the comments suggest that is a **bad** idea and I agree, but as the logging happens in there rather than in any of the protection method classes, I can't override the behaviour the preferred way.

The other implementation options I've thought of:
* push all the logging down into the classes in ProtectionMethods and then I re-implement NullSession in my own app code as QuietNullSession
* add a QuietNullSession to ProtectionMethods

I prefer it to be a config option because that way I don't have to worry about keeping my CSRF stuff in sync with rails over time, because honestly I'm unlikely to, and then _security_ will happen to me.