Changes: Ask User to authenticate the login request

API side: 
```
+    authenticate : function (username, password, cb) {
+      Model.authenticate(username, password, function(result) {
+        cb (null, result);
+      });
'''

And in the model:
```
+userSchema.statics.authenticate = function(username, password, cb) {
+  // TODO: This is the simplest implementation, just return true to say that it is authenticated 
+  cb(true);
+};

'''