Rename entry points for annotator and sidebar bundles to 'index.js'

Rename the entry points to the more conventional 'index.js' for the
benefit of new visitors to the codebase and also for consistency across
the different bundles that make up the client.

gulpfile.js

@@ -107,12 +107,12 @@ var appBundles = [{
   // The sidebar application for displaying and editing annotations.
   name: 'app',
   transforms: ['babel', 'coffee'],
-  entry: './src/sidebar/app',
+  entry: './src/sidebar/index',
 },{
   // The annotation layer which handles displaying highlights, presenting
   // annotation tools on the page and instantiating the sidebar application.
   name: 'injector',
-  entry: './src/annotator/main',
+  entry: './src/annotator/index',
   transforms: ['babel', 'coffee'],
 }];