Contribution back to BouncyCastle

Since our products OpenPGP Library for Java and OpenPGP Library for .NET are based on the open source BouncyCastle cryptographic library, it was time for us to make a contribution to this great project.

Although our first contribution is very small, it provides capabilities for generation of OpenPGP revocation certificates, key revocation and assigning designated revokers. The contribution consists of the classes RevocationReason and RevocationKey and enums RevocationReasonTag and RevocationKeyTag in the namespace Org.BouncyCastle.Bcpg, including a small change in PgpSignatureSubpacketGenerator to support them.

Special thanks to Peter Dettman from LockBox Labs for accepting our contribution and applying it to the Java branch as well.