dkim_sign: added config/dkim/+domain support
dkim: completed per-domain dkim key use
update DKIM doc.md, add comments in 'dns' file
dkim: check for higher level DKIM keys when signing

for example, when a message is being sent from mail.example.com, also
check for an example.com signing key.
dkim docs: formatting fixes
dkim docs: clarify single use config
dkim: removed sync fs calls
dkim: replaced Array() with []
removed fs.sync methods
dkim: remove extra checks
Merge pull request #403 from msimerson/dkim

DKIM per-domain signing support
add Authentication-Results header (RFC 5451)
Auth-Res: added SPF results
auth: added auth method to A-R header
lookup_rdns: added iprev to Auth-Results header
connection: store auth results in transaction

when a transaction is available. Else, default to connection
move old Auth-Results to Original-Auth-Results
spf: moved auth_results processing out of switch

so that it's always added to auth_results
previously, when SPF failed, it wasn't added.
When combined with a delay_deny plugin, the connection would ultimately pass without SPF results saved
connection: made code more linear

make it easier to read and grok
Merge branch 'authentication-results-rfc5451' of github.com:msimerson/Haraka into authentication-results-rfc5451