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/PreKeyResponseItemV2.java

@@ -0,0 +1,64 @@
+/**
+ * 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;
+
+public class PreKeyResponseItemV2 {
+
+  @JsonProperty
+  private long deviceId;
+
+  @JsonProperty
+  private int registrationId;
+
+  @JsonProperty
+  private DeviceKey deviceKey;
+
+  @JsonProperty
+  private PreKeyV2 preKey;
+
+  public PreKeyResponseItemV2() {}
+
+  public PreKeyResponseItemV2(long deviceId, int registrationId, DeviceKey deviceKey, PreKeyV2 preKey) {
+    this.deviceId       = deviceId;
+    this.registrationId = registrationId;
+    this.deviceKey      = deviceKey;
+    this.preKey         = preKey;
+  }
+
+  @VisibleForTesting
+  public DeviceKey getDeviceKey() {
+    return deviceKey;
+  }
+
+  @VisibleForTesting
+  public PreKeyV2 getPreKey() {
+    return preKey;
+  }
+
+  @VisibleForTesting
+  public int getRegistrationId() {
+    return registrationId;
+  }
+
+  @VisibleForTesting
+  public long getDeviceId() {
+    return deviceId;
+  }
+}