add three checks:
1 signer workingdirectory should exist (if not specified, default directory should exist)
2 enforcer workingdirectory should exist (if not specified, default directory should exist)
3 signer workingdirectory shouldnot be the same as the one of enforcer
Merge pull request #20 from massonjiang/feature/OPENDNSSEC-513

OPENDNSSEC-513