Render group updates

Not pretty, but it works. Also allows for later localization.
Copy/behavior is borrowed from the Android client.

Closes #104
Fixes #65

test/views/group_update_view_test.js

@@ -0,0 +1,16 @@
+describe('GroupUpdateView', function() {
+  it('should show new group members', function() {
+    var view = new Whisper.GroupUpdateView({model: {joined: ['Alice', 'Bob']}}).render();
+    assert.match(view.$el.text(), /Alice.*Bob.*joined the group/);
+  });
+
+  it('should note updates to the title', function() {
+    var view = new Whisper.GroupUpdateView({model: {name: 'New name'}}).render();
+    assert.match(view.$el.text(), /Title is now 'New name'/);
+  });
+
+  it('should say "Updated the group"', function() {
+    var view = new Whisper.GroupUpdateView({model: {avatar: 'New avatar'}}).render();
+    assert.match(view.$el.text(), /Updated the group/);
+  });
+});