[WIP] 'Zend\Form\FormManager' implementation and 'EventManager' integration

I opened an issue (#4228) to discuss some ideas I had, and now I have some code I wrote. This PR is supposed to provide the following:

- [x] Implementation of `Zend\Form\FormManager`:
    This class will manage anything related to `Zend\Form`, like `FormElementManager`, `FormFactory`, `InputFilterFactory` and possibly `FormObjectManager` - see #4100.

- [x] Creation of `Zend\Form\FormFactory` to replace `Zend\Form\Factory`:
    The intention of this is that the factory would use `FormManager` to retrieve its dependencies. The `Zend\Form\Factory` would be deprecated.

- [ ] Create a `ModuleManager` listener:
    We would deprecate `form_elements` key and `getFormElementConfig` method and provide a `form_manager` key (similar with the one presented by @stephanoff in #4100), and a method `getFormConfig()`.

    ```PHP
    public function getFormConfig()
    {
        return array(
            'forms' => ( /* Especifications for Zend\Form\FormAbstractServiceFactory */ ),
            'element_manager' => (),
            'object_manager'  => (), // plz refer to #4100
        );
    }
    ```
    Or in a config file:

    ```PHP
    return array(
        'form_manager' => array(
            'forms' => (),
            'elements' => (), // instead of 'element_manager'
            'objects'  => (), // instead of 'object_manager'
        ),
    );
    ```

    Something like this, let me know if you have suggestions.

- [ ] Implementation of a `Zend\EventManager` integration:
    This integration would be added as a **soft** dependency that could be activated via a key in the configuration (similar with the one presented by @DASPRiD in #4187):

    ```PHP
    return array(
        'form_manager' => array(
            'event_manager_enabled' => true,
        ),
    );
    ```
    The idea of this integration is to provide something like the following examples:

    ```PHP
    // In a 'product' fieldset
    $this->add(array(
        'name' => 'isSale',
        'type' => 'Checkbox',
    ));

    $this->add(array(
        'name' => 'salePrice',
        'event_listeners' => array( // array of events to be attached to the EventManager
            array(
                'event'    => 'validate', // Would be triggered when isValid() gets called
                'callback' => function ($e) {
                    $form = $e->getForm();

                    // If 'isSale' is NOT checked 'salePrice' should not be requiered
                    if (!$form->get('product')->get('isSale')->isChecked()) {
                        $form->getInputFilter()->get('product')->get('salePrice')->setRequired(false);
                    }
                },
                'priority' => 25,
            ),
            array(
                'event'    => 'prepare', // Would be triggered when prepare() gets called
                'callback' => function ($e) {
                    $form = $e->getForm();

                    if (!$form->get('product')->get('isSale')->isChecked()) {
                        // This could be done in the 'validate' event, but it's here just for demonstration
                        $form->get('product')->get('salePrice')->setAttribute('disabled', 'disabled');
                    }
                },
            ),
        ),
    ));
    ```

    **Note: _The `EventManager` Integration should probably go in another PR, but I'm just showing where all this came from_**