code: function Card(front, back) {
    ui.Emitter.call(this);
    this._front = front || $('<p>front</p>');
    this._back = back  || $('<p>back</p>');
    this.template = html;
    this.render();
  }
docs: Initialize a new `Card` with content
  for face `front` and `back`.
  
  Emits "flip" event.
  
  @param {Mixed} front
  @param {Mixed} back
  @api public