Gradle build files for inclusion into OpenKeychain
Expose S2K in PGPSecretKey
Add static values for GNU protection modes in PGP S2K class

Conflicts:
	pg/src/main/java/org/spongycastle/bcpg/S2K.java
OpenPGP: Dont write version header by default
add PGPPublicKey.getKeySignatures method
add addSubkeyBindingCertification method to PGPPublicKey
Fix version to 1.51