Add a TransactionBroadcaster abstraction to PeerGroup.

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

@@ -67,7 +67,7 @@ import static com.google.common.base.Preconditions.checkState;
  * when finished. Note that not all methods of PeerGroup are safe to call from a UI thread as some may do
  * network IO, but starting and stopping the service should be fine.</p>
  */
-public class PeerGroup extends AbstractIdleService {
+public class PeerGroup extends AbstractIdleService implements TransactionBroadcaster {
     private static final int DEFAULT_CONNECTIONS = 4;
 
     private static final Logger log = LoggerFactory.getLogger(PeerGroup.class);

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

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.bitcoin.core;
+
+import com.google.common.util.concurrent.ListenableFuture;
+
+/**
+ * A general interface which declares the ability to broadcast transactions. This is implemented
+ * by {@link com.google.bitcoin.core.PeerGroup}.
+ */
+public interface TransactionBroadcaster {
+    /** Broadcast the given transaction on the network */
+    public ListenableFuture<Transaction> broadcastTransaction(final Transaction tx);
+}