Add support for SPF record type

The implementation of SPF is the same as TXT
> This document defines a new DNS RR of type SPF, code 99.  The format of this type is identical to the TXT RR [RFC1035].  For either type, the character content of the record is encoded as [US-ASCII].
... from http://www.ietf.org/rfc/rfc4408.txt

SPF values are exposed via `ans.spf`.