Introduce V2 API for PreKey updates and requests.

1) A /v2/keys controller.

2) Separate wire protocol PreKey POJOs from database PreKey
   objects.

3) Separate wire protocol PreKey submission and response POJOs.

4) Introduce a new update/response JSON format for /v2/keys.

src/main/java/org/whispersystems/textsecuregcm/entities/PreKeyResponseV2.java

@@ -0,0 +1,48 @@
+/**
+ * Copyright (C) 2014 Open Whisper Systems
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.whispersystems.textsecuregcm.entities;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.common.annotations.VisibleForTesting;
+
+import java.util.List;
+
+public class PreKeyResponseV2 {
+
+  @JsonProperty
+  private String identityKey;
+
+  @JsonProperty
+  private List<PreKeyResponseItemV2> devices;
+
+  public PreKeyResponseV2() {}
+
+  public PreKeyResponseV2(String identityKey, List<PreKeyResponseItemV2> devices) {
+    this.identityKey = identityKey;
+    this.devices     = devices;
+  }
+
+  @VisibleForTesting
+  public String getIdentityKey() {
+    return identityKey;
+  }
+
+  @VisibleForTesting
+  public List<PreKeyResponseItemV2> getDevices() {
+    return devices;
+  }
+}