HDW: Add an AUTHENTICATION key purpose that's mapped to the internal chain.

core/src/main/java/com/google/bitcoin/wallet/DeterministicKeyChain.java

@@ -342,6 +342,7 @@ public class DeterministicKeyChain implements EncryptableKeyChain {
                     index = issuedExternalKeys;
                     parentKey = externalKey;
                     break;
+                case AUTHENTICATION:
                 case CHANGE:
                     issuedInternalKeys += numberOfKeys;
                     index = issuedInternalKeys;

core/src/main/java/com/google/bitcoin/wallet/KeyChain.java

@@ -38,10 +38,11 @@ public interface KeyChain {
     /** Returns true if the given key is in the chain. */
     public boolean hasKey(ECKey key);
 
-    enum KeyPurpose {
+    public enum KeyPurpose {
         RECEIVE_FUNDS,
         CHANGE,
-        REFUND
+        REFUND,
+        AUTHENTICATION
     }
 
     /** Obtains a number of key/s intended for the given purpose. The chain may create new key/s, derive, or re-use an old one. */