Prevent notice about $post being a "non-object"

There are situation when $post is not a WP_Post Object. I experienced this in a 0 search result case.
This causes a Notice-Warning when „define('WP_DEBUG', true)“:

„Notice: Trying to get property of non-object in …wp-content/plugins/wp-facebook-open-graph-protocol/wp-facebook-ogp.php on line 159 
Notice: Trying to get property of non-object in …wp-includes/post-template.php on line 29 
Notice: Trying to get property of non-object in …wp-content/plugins/wp-facebook-open-graph-protocol/wp-facebook-ogp.php on line 42“
Move from [] to array()

My staging system did not like the [] even though development did …
Remove unused whitespaces
Add wpfbogp_fallback_img filter
Allow the options to be set per multisite

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
Check for relative URLs in post thumbnails images #33
Update documentation for functions.

All functions follow this new way of formatting the documentation.
flip img array, fix relative path in img names, fix string in arrays, update screenshot, update readme
Merge branch 'master' into multisite_support

Conflicts:
	wp-facebook-ogp.php
Merge branch 'master' of https://github.com/chuckreynolds/WPFBOGP

Conflicts:
	wp-facebook-ogp.php