Correct the DNSKEY flag values

See https://www.iana.org/assignments/dnskey-flags/dnskey-flags.xhtml

Elsewhere in the code 256 (1 << 8) is used numerically for ZONE, so nothing breaks for now.
Merge pull request #122 from FiloSottile/patch-1

Correct the DNSKEY flag values
Add extra (failing) sign/verify test
Cleanup the test a bit
Lowercase the Test to test
Update documentation references for EDNS0 and EDNS0_SUBNET.

Only EDNS0 is defined in RFC 6891 and EDNS0_SUBNET is actually just a draft.
Merge pull request #123 from cpatulea/master

Update documentation references for EDNS0 and EDNS0_SUBNET.
Merge remote-tracking branch 'miekg/master'