Use signed 64-bit values for TransactionOutput.value.

The reference client (stupidly) does this, and we were incorrectly
de-serializing values from the reference client.

core/src/main/java/com/google/bitcoin/core/Message.java

@@ -410,6 +410,11 @@ public abstract class Message implements Serializable {
         return new Sha256Hash(hash);
     }
 
+    long readInt64() {
+        long u = Utils.readInt64(bytes, cursor);
+        cursor += 8;
+        return u;
+    }
 
     BigInteger readUint64() {
         // Java does not have an unsigned 64 bit type. So scrape it off the wire then flip.

core/src/main/java/com/google/bitcoin/core/TransactionOutput.java

@@ -131,7 +131,11 @@ public class TransactionOutput extends ChildMessage implements Serializable {
     }
 
     protected void parseLite() {
-        value = readUint64();
+        // TODO: There is no reason to use BigInteger for values, they are always smaller than 21 million * COIN
+        // The only reason to use BigInteger would be to properly read values from the reference implementation, however
+        // the reference implementation uses signed 64-bit integers for its values as well (though it probably shouldn't)
+        long outputValue = readInt64();
+        value = BigInteger.valueOf(outputValue);
         scriptLen = (int) readVarInt();
         length = cursor - offset + scriptLen;
     }

core/src/main/java/com/google/bitcoin/core/Utils.java

@@ -238,6 +238,17 @@ public class Utils {
                 ((bytes[offset++] & 0xFFL) << 16) |
                 ((bytes[offset] & 0xFFL) << 24);
     }
+    
+    public static long readInt64(byte[] bytes, int offset) {
+        return ((bytes[offset++] & 0xFFL) << 0) |
+               ((bytes[offset++] & 0xFFL) << 8) |
+               ((bytes[offset++] & 0xFFL) << 16) |
+               ((bytes[offset++] & 0xFFL) << 24) |
+               ((bytes[offset++] & 0xFFL) << 32) |
+               ((bytes[offset++] & 0xFFL) << 40) |
+               ((bytes[offset++] & 0xFFL) << 48) |
+               ((bytes[offset] & 0xFFL) << 56);
+    }
 
     public static long readUint32BE(byte[] bytes, int offset) {
         return ((bytes[offset + 0] & 0xFFL) << 24) |