Only sign records from within our zone.

Closes #40
Be more verbose on what address we are listening.
Add Lookup function.

Allows for remote lookups for external name when encountering
internal cnames.
Extra CNAME resolving.

If a cname chain encounters an external name try to lookup that name
too. This is expected by applications when using SkyDNS as an recursor.
Add tests for external CNAME resolution.