Cherry pick and pre-bind underscore methods (borrowed from backbone)

wrapper.js

var Url = function (url) {
  this._url = url;
};

_.extend(Url.prototype, {
  /* more prescriptive methods */  
});

var methods = ['forEach', 'each', 'map', 'find', 'detect', 'filter',
  'select', 'reject', 'every', 'all', 'some', 'any', 'include',
  'contains', 'invoke', 'toArray', 'first', 'initial', 'rest',
  'last', 'without', 'isEmpty', 'pluck'];

_.each(methods, function(method) {
  Url.prototype[method] = function() {
    var fragments = this._url.split('/');
    
    var args = [fragments].concat(_.toArray(arguments));
    
    return _[method].apply(_, args);
  };
});