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.

src/org/thoughtcrime/securesms/util/GroupUtil.java

@@ -0,0 +1,27 @@
+package org.thoughtcrime.securesms.util;
+
+import org.whispersystems.textsecure.util.Hex;
+
+import java.io.IOException;
+
+public class GroupUtil {
+
+  private static final String ENCODED_GROUP_PREFIX = "__textsecure_group__!";
+
+  public static String getEncodedId(byte[] groupId) {
+    return ENCODED_GROUP_PREFIX + Hex.toStringCondensed(groupId);
+  }
+
+  public static byte[] getDecodedId(String groupId) throws IOException {
+    if (!isEncodedGroup(groupId)) {
+      throw new IOException("Invalid encoding");
+    }
+
+    return Hex.fromStringCondensed(groupId.split("!", 2)[1]);
+  }
+
+  public static boolean isEncodedGroup(String groupId) {
+    return groupId.startsWith(ENCODED_GROUP_PREFIX);
+  }
+
+}

src/org/thoughtcrime/securesms/util/NumberUtil.java

@@ -34,6 +34,12 @@ public class NumberUtil {
     return PhoneNumberUtils.isWellFormedSmsAddress(number) || isValidEmail(number);
   }
 
+  public static boolean isValidSmsOrEmailOrGroup(String number) {
+    return PhoneNumberUtils.isWellFormedSmsAddress(number) ||
+        isValidEmail(number) ||
+        GroupUtil.isEncodedGroup(number);
+  }
+
   public static String filterNumber(String number) {
     if (number == null) return null;