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/util/Hex.java

@@ -16,6 +16,8 @@
  */
 package org.whispersystems.textsecure.util;
 
+import java.io.IOException;
+
 /**
  * Utility for generating hex dumps.
  */
@@ -43,6 +45,36 @@ public class Hex {
     return buf.toString();
   }
 
+  public static String toStringCondensed(byte[] bytes) {
+    StringBuffer buf = new StringBuffer();
+    for (int i=0;i<bytes.length;i++) {
+      appendHexChar(buf, bytes[i]);
+    }
+    return buf.toString();
+  }
+
+  public static byte[] fromStringCondensed(String encoded) throws IOException {
+    final char[] data = encoded.toCharArray();
+    final int    len  = data.length;
+
+    if ((len & 0x01) != 0) {
+      throw new IOException("Odd number of characters.");
+    }
+
+    final byte[] out = new byte[len >> 1];
+
+    // two characters form the hex value.
+    for (int i = 0, j = 0; j < len; i++) {
+      int f = Character.digit(data[j], 16) << 4;
+      j++;
+      f = f | Character.digit(data[j], 16);
+      j++;
+      out[i] = (byte) (f & 0xFF);
+    }
+
+    return out;
+  }
+
   public static String dump(byte[] bytes) {
     return dump(bytes, 0, bytes.length);
   }