Add flash messenger view helper

[weierophinney]

The flash messenger view helper in zend-view depends on the MVC plugin. We should move it into this repository, and add view helper configuration in the Module class to define the view helper service for it to point to the one in this repository.

This will eventually enable the ability to remove it from zend-view for a v3 release of that component.

[adamlundrigan]

How's this for a plan of attack?

Addition this repository:

• Copy Zend\View\Helper\FlashMessenger to Zend\Mvc\Plugin\FlashMessenger\View\Helper\FlashMessenger (+test class)
• Copy Zend\View\Helper\Service\FlashMessengerFactory to Zend\Mvc\Plugin\FlashMessenger\View\Helper\Service\FlashMessengerFactory (+test class)
• Copy flash-messenger.md over from zend-view
• Update Module::getConfig to register the view helper

Removal from zend-view:

• Drop Zend\View\Helper\FlashMessenger (+test class)
• Drop Zend\View\Helper\Service\FlashMessengerFactory (+test class)
• Drop FlashMessenger from zend-view's HelperPluginManager
• Replace existing docs with link to new one in this repository