code: function(context, expression, onChange, callbackContext) {
      var observer = this.createObserver(expression, onChange, callbackContext || context);
      observer.bind(context);
      return observer;
    }
docs: Observes any changes to the result of the expression on the context object and calls the callback.
  @param {Object} context The context to bind the expression against
  @param {String} expression The expression to observe
  @param {Function} onChange The function which will be called when the expression value changes
  @return {Observer} The observer created