Fixing prepended `NotEmpty` validator

When using `InputFilter` standalone, [this](https://github.com/zendframework/zf2/blob/master/library/Zend/InputFilter/Input.php#L375) is broken because [this line](https://github.com/zendframework/zf2/blob/master/library/Zend/Validator/ValidatorChain.php#L186) calls `ValidatorPluginManager`, and this class inherits `AbstractPluginManager`, that is `suggested` at [composer.json](https://github.com/zendframework/zf2/blob/master/library/Zend/InputFilter/composer.json#L26), not required.

Then the following error is thrown:
`class Zend\ServiceManager\AbstractPluginManager not found.`

I just changed the `prependByName` method (that uses PluginManager) to `prependValidator`, instantiating the validator by hand.