DKIM updates against smf's PR #695

I intended to open a PR against @smfreegard's branch with DKIM in it, and then discovered that his PR #695 was against a version of smfreegard/master that no longer exists. So I couldn't.

Not having a foggy clue how Steve plans to continue work on this PR, this seemed like a reasonable way to attempt contributing. This PR starts with Steve's commits in PR #695 and applies jshint fixups and a few suggestions made by @baudehlo. I commented inline on PR #695 on the issues I resolved.

I pushed a small change to my DMARC plugin and now it consumes the DKIM output from this plugin. Nice.

Since deploying this version (~2 hours ago), I have not seen a dkim_verify timeout. But I only had 85 timeouts yesterday and traffic is low in the evening, so the jury is still out.