Don't clone target function

_.cloneDeep() of a function just returns {} in Lodash 3+, so the function is lost if you do:
```
'GET /foo': {target: function(req, res){...}}
```

lib/router/bind.js

@@ -56,7 +56,7 @@ function bind( /* path, target, verb, options */ ) {
   // If target is an object with a `target`, pull out the rest
   // of the keys as route options and then bind the target.
   else if (_.isPlainObject(target) && target.target) {
-    var _target = _.cloneDeep(target.target);
+    var _target = target.target;
     options = _.merge(options, _.omit(target, 'target'));
     bind.apply(this, [path, _target, verb, options]);
   }