Remove hard dependency on ServiceManager in Feed

- Created Zend\Feed\Reader\ExtensionManagerInterface and
  Zend\Feed\Writer\ExtensionManagerInterface. Default ExtensionManager
  implementations implement these while extending AbstractPluginManager.
- You can implement the interface and inject your implementation as the default
  ExtensionManager if desired.
Use decorator pattern

- PHP 5.3.3 does not like it when you implement multiple interfaces that define
  the same method, even if the signatures are identical.
- Workaround is to implement a decorator.
Same thing for Writer

- Make ExtensionManager a decorator
Merge branch 'feature/feed-segregated-manager-interface' into develop

Forward port #4396

Conflicts:
	library/Zend/Feed/composer.json