Add material tree example (#28)

Add sanity check test for tree example. 
Remove block element styling that makes source code links clickable area stretch across the page.

Author: nshahan

lib/app_component.css

@@ -4,12 +4,14 @@
  * BSD-style license that can be found in the LICENSE file.
  */
 
-.source-link:link, .source-link:visited, .source-link:active {
-  display: block;
-  padding-bottom: 10px;
+ul {
+  list-style-type: none;
 }
 
-.source-link:hover {
-  display: block;
-  padding-bottom: 10px;
+li {
+  padding: 5px 0;
+}
+
+ul.source-links {
+  padding: 0;
 }

lib/app_component.dart

@@ -21,6 +21,7 @@ import 'src/material_spinner_demo/material_spinner_demo.dart';
 import 'src/material_tab_demo/material_tab_demo.dart';
 import 'src/material_toggle_demo/material_toggle_demo.dart';
 import 'src/material_tooltip_demo/material_tooltip_demo.dart';
+import 'src/material_tree_demo/material_tree_demo.dart';
 import 'src/material_yes_no_buttons_demo/material_yes_no_buttons_demo.dart';
 import 'src/reorder_list_demo/reorder_list_demo.dart';
 import 'src/scorecard_demo/scorecard_demo.dart';
@@ -47,6 +48,7 @@ import 'src/scorecard_demo/scorecard_demo.dart';
     MaterialTabDemoComponent,
     MaterialToggleDemoComponent,
     MaterialTooltipDemoComponent,
+    MaterialTreeDemoComponent,
     MaterialYesNoButtonsDemoComponent,
     ReorderListDemoComponent,
     ScorecardDemoComponent,