encoding: In OID, make toString, operator== and operator!= const. Refs #1091.
security: In CertificateSubjectDescription, make toDer and getOidString const. Refs #1091.
security: In CertificateExtension, make toDer and derDerBlob const. Refs #1091.
Merge branch 'issue/1091-const-CertificateSubjectDescription-getOidString'. Refs #1091.
http://redmine.named-data.net/issues/1091