This patch adds some tooling and scripts to generate test coverage
information.
Unfortunately, this involves some hacks as Go does not have support for
generating coverage-enabled binaries, or merging coverage reports; but
overall it's not very intrusive.
This patch adds a package for evaluating SPF, as defined by RFC 7208
(https://tools.ietf.org/html/rfc7208).
It doesn't implement 100% of the RFC, but it coves enough to handle the
most common cases, and will fail open on the others.