Rename global variable 'Chat' to 'Snapdrop'

Initially a lot of code was copied from another project. This lead to confusing naming conventions.

app/elements/invitation-link/invitation-link-behavior.html

@@ -1,7 +1,7 @@
 <script>
 'use strict';
-window.Chat = window.Chat || {};
-Chat.InvitationLinkBehavior = {
+window.Snapdrop = window.Snapdrop || {};
+Snapdrop.InvitationLinkBehavior = {
     properties: {
         contact: {
             type: String

app/elements/invitation-link/invitation-link.html

@@ -23,7 +23,7 @@
     'use strict';
     Polymer({
         is: 'invitation-link',
-        behaviors: [Chat.InvitationLinkBehavior]
+        behaviors: [Snapdrop.InvitationLinkBehavior]
     });
     </script>
 </dom-module>