[#5] Create basic Cert
[#5] Create Signature class
[#5] Implement Cert::hasSigned
[#5] Expose OIDs from phpseclib

These are accessible from KG\DigiDoc\X509\Asn1::getValueForOid().
[#5] Implement Response::isSignedBy()
[#5] Verify OCSP response signatures
[#5] Fix a test relying on missing files

No need to read certs from the disk, they're now embedded in the test.
[#5] Fix CertTest failing in PHP <=5.3
Merge branch '5-verify-ocsp-responses'