Service Manager initializers improvements

Functionality for binding service manager initializers to specific services or class types, so that they can be executed only for objects "of interest". For example:
```php
$serviceManager->addInitializer('My\Initializer', true, 'FooService');
```
or when targeting class types:
```php
$serviceManager->addInitializer('My\Initializer', true, 'Foo\SomeServiceAwareInterface');
```