make amnesiadb compatible with netflux

storage/amnesia.js

@@ -18,6 +18,7 @@ module.exports.create = function(conf, cb){
 
     var db=[],
         index=0;
+
     cb({
         message: function(channelName, content, cb){
             var val = {
@@ -27,17 +28,18 @@ module.exports.create = function(conf, cb){
                 time: new Date().getTime(),
             };
             db.push(val);
-            cb();
+            if (cb) { cb(); }
         },
-        getMessages: function(channelName, cb){
+        getMessages: function(channelName, handler, cb){
             db.sort(function(a,b){
                 return a.id - b.id;
             });
             db.filter(function(val){
                 return val.chan === channelName;
             }).forEach(function(doc){
-                cb(doc.msg);
+                handler(doc.msg);
             });
+            if (cb) { cb(); }
         },
     });
 };