Merge Whisper.Messages into Whisper Threads

Eliminates the global Whisper.Messages object and consolidates shared
send/receive logic in Whisper.Threads.

To the latter end, note that the decrypted array buffer on an attachment
pointer is now named data instead of decrypted, in order to match the
format of outgoing attachments presented by
FileReader.readAsArrayBuffers and let us use the same handler to base64
encode them.

js/helpers.js

@@ -269,7 +269,7 @@ window.textsecure.subscribeToPush = function(message_callback) {
                 var handleAttachment = function(attachment) {
                     return textsecure.api.getAttachment(attachment.id.toString()).then(function(encryptedBin) {
                         return textsecure.protocol.decryptAttachment(encryptedBin, attachment.key.toArrayBuffer()).then(function(decryptedBin) {
-                            attachment.decrypted = decryptedBin;
+                            attachment.data = decryptedBin;
                         });
                     });
                 };