Use F4 exponent for RSA key generation.

(As per Colin Percival's advice in his BSDCan presentation.)

src/main/java/com/wesabe/grendel/openpgp/AsymmetricAlgorithm.java

@@ -44,7 +44,7 @@ public AlgorithmParameterSpec getAlgorithmParameterSpec() {
 	RSA(		"RSA",			PublicKeyAlgorithmTags.RSA_GENERAL) {
 		@Override
 		public AlgorithmParameterSpec getAlgorithmParameterSpec() {
-			return new RSAKeyGenParameterSpec(2048, RSAKeyGenParameterSpec.F0);
+			return new RSAKeyGenParameterSpec(2048, RSAKeyGenParameterSpec.F4);
 		}},
 
 	/**