Add wordpress multisite support

We misunderstood each other in https://github.com/chuckreynolds/WPFBOGP/pull/2 and I only just realized it …
The solution with filters is not really working for this usecase since they only allow modifying those settings that are set to a value already. I was looking for a way to set the settings for all blogs at once.

Here is my solution.

Tobias

------

UseCase: Set the plugin data via a separate plugin and only once for a
multisite.

* move the get_options and delete_options to separate methods
* allow multisite settings to overwrite blog settings
* provide a way to hide the menu item for those cases

Example Plugin to set the multisite options:

```php
<?php
/**
* @package bp_wpfbogp_conguration
* @version 1.0
*/
/*
Plugin Name: WP Facebook Open Graph protocol - betterplace.org defaults
Plugin URI:
Description: This sets the facebook ap id and fallback image for all
multisites.
Author: Tobias Jordans
Version: 1.0
Author URI:
*/

register_activation_hook( __FILE__, 'bp_wpfbogp_conguration' );

function bp_wpfbogp_conguration() {
$wpfbogp_set_options = array(
'wpfbogp_admin_ids' => false, # we need to set this to prevent
errors b/c of missing array index
'wpfbogp_force_fallback' => false, # we need to set this to prevent
errors b/c of missing array index
'wpfbogp_app_id' => '123',
'wpfbogp_fallback_img' =>
'http://www.betterplace.org/images/layout-logo.betterplace.png',
'wpfbogp_hide_page' => false
);

update_site_option( 'wpfbogp', $wpfbogp_set_options );
}
```