Add capability guards for the database functionality.

js/db.js

@@ -24,6 +24,9 @@ define(
                     }
                 }
             },
+            Capable: function() {
+                return typeof indexedDB != "undefined";
+            },
             // Turn on/off debug logging.
             ToggleDebug: function() {
                 this.debug = !this.debug;

js/tagging.js

@@ -12,7 +12,7 @@ define(
         // callbacks.error is expected to be a function that takes
         // an IDBRequest as an argument.
         UpdateTags: function(ref, tags, callbacks) {
-            db.Transaction(
+            db.Capable() && db.Transaction(
                 [tagStore], function(trans) {
                     db.Update(
                         'tags', ref.toString(),
@@ -27,7 +27,7 @@ define(
         // callbacks.error is expected to be a function that takes
         // an IDBRequest as an argument.
         GetTags: function(ref, callbacks) {
-            db.Get(tagStore, ref.toString(),
+            db.Capable() && db.Get(tagStore, ref.toString(),
                    {success: function(req) {
                         var data = req.target.result;
                         // TODO suppport book and chapter indexes for