Support encrypted transport, properly handle multiple recipients.

1) Add encryption support for the transport layer.  This obscures
   metadata from the push messaging provider.

2) Better support the direction multiple destination messages is
   headed (one unique message per recipient).

library/protobuf/IncomingPushMessageSignal.proto

@@ -0,0 +1,19 @@
+package textsecure;
+
+option java_package = "org.whispersystems.textsecure.push";
+option java_outer_classname = "PushMessageProtos";
+
+message IncomingPushMessageSignal {
+  optional uint32 type = 1;
+  optional string source = 2;
+  repeated string destinations = 3;
+  optional bytes message = 4;
+
+  message AttachmentPointer {
+    optional string contentType = 1;
+    optional string key = 2;
+  }
+
+  repeated AttachmentPointer attachments = 5;
+  optional uint64 timestamp = 6;
+}