Add a post installation hook

This adds a hook directly after Easy Digital Downloads has been installed, allowing theme and plugin devs to hook into the activation event. An array of the newly created pages is sent as the only parameter. 

Below is a quick example showing how I would use this in a theme development context: 

```php
function my_edd_after_install( $activation_pages ) {
    foreach ( $activation_pages as $page ) {
        update_post_meta( $page, '_wp_page_template', 'page-fullwidth.php' );
    }
}
add_action( 'edd_after_install', 'my_edd_after_install' );
```

Cheers, 
Eric