Validation: add expect executor

[joshjung]

Target Code:

import {expects} from 'ringa';

controller.addListener('event', [
  expect('someProperty', 'someOtherProperty, 'blah'),
  MyExecutor
]);

Will throw an error and kill the thread if the ringaEvent.detail does not have these properties. This is a way to clarify at the beginning of a rather large executor tree what the developer expects to be available.