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 pull request #409 from msimerson/authentication-results-rfc5451

add Authentication-Results header (RFC 5451)