Rename some things to be a little more semantic

The 'sender' field actually holds the recipient for outgoing
messages. Rename that field to 'person', indicating the 2nd
party generically.

Also decouples the thread name from thread recipients at the
view layer, in preparation for group support.

js/models/messages.js

@@ -11,7 +11,7 @@ var Whisper = Whisper || {};
 
     addIncomingMessage: function(decrypted) {
       Whisper.Messages.add({
-        sender: decrypted.pushMessage.source,
+        person: decrypted.pushMessage.source,
         group: decrypted.message.group,
         body: decrypted.message.body,
         type: 'incoming',
@@ -19,9 +19,9 @@ var Whisper = Whisper || {};
       }).save();
     },
 
-    addOutgoingMessage: function(messageProto, sender) {
+    addOutgoingMessage: function(messageProto, recipients) {
       Whisper.Messages.add({
-        sender: sender,
+        person: recipients[0], // TODO: groups
         body: messageProto.body,
         type: 'outgoing',
         timestamp: new Date().getTime()