Add option to mark properties as private API

[sandersky]

We should be able to mark certain properties as private, complaining when the property is set by the consumer when initializing an instance. For example a consumer of Ember.Component should not be passing in isDestroyed or isDestroying. We should allow a component to do something like:

propTypes: {
  isDestroyed: PropTypes.bool({private: true}),
  isDestroying: PropTypes.bool({private: true})
}