Support for multi-device.

1) In addition to the Recipient interface, there is now
   RecipientDevice.  A Recipient can have multiple corresponding
   RecipientDevices.  All addressing is done to a Recipient, but
   crypto sessions and transport delivery are done to
   RecipientDevice.

2) The Push transport handles the discovery and session setup
   of additional Recipient devices.

3) Some internal rejiggering of Groups.

library/src/org/whispersystems/textsecure/push/PushAddress.java

@@ -0,0 +1,32 @@
+package org.whispersystems.textsecure.push;
+
+import android.content.Context;
+
+import org.whispersystems.textsecure.directory.Directory;
+import org.whispersystems.textsecure.storage.RecipientDevice;
+
+public class PushAddress extends RecipientDevice {
+
+  private final String e164number;
+  private final String relay;
+
+  private PushAddress(long recipientId, String e164number, int deviceId, String relay) {
+    super(recipientId, deviceId);
+    this.e164number  = e164number;
+    this.relay       = relay;
+  }
+
+  public String getNumber() {
+    return e164number;
+  }
+
+  public String getRelay() {
+    return relay;
+  }
+
+  public static PushAddress create(Context context, long recipientId, String e164number, int deviceId) {
+    String relay = Directory.getInstance(context).getRelay(e164number);
+    return new PushAddress(recipientId, e164number, deviceId, relay);
+  }
+
+}