Validate/reformat phone numbers in contact syncs

Turns out there's no garauntee that Android will send us contact info
with phone numbers in e164 format. When that happens, we fail to update
the correct contact. Fix by performing validation on the incoming number
before attempting to merge changes to the name, avatar, or color.

Fixes #903

js/background.js

@@ -128,14 +128,22 @@
 
     function onContactReceived(ev) {
         var contactDetails = ev.contactDetails;
-        ConversationController.create({
+
+        var c = new Whisper.Conversation({
             name: contactDetails.name,
             id: contactDetails.number,
             avatar: contactDetails.avatar,
             color: contactDetails.color,
             type: 'private',
             active_at: Date.now()
-        }).save();
+        });
+        var error;
+        if (error = c.validateNumber()) {
+          console.log(error);
+          return;
+        }
+
+        ConversationController.create(c).save();
     }
 
     function onGroupReceived(ev) {