Add Watermark feature

[emptynick]

Maybe you dont want watermarks in your plugin, but its actually really easy to implement.
This would require a settings-page for the backend (if settings aren't taken from config.php).
Here are my suggestions for this feature:

• Set watermark position
• Set watermark size (in percent)
• Set picture for watermark (small and big) with a threshold for small and big pictures

Later on I will push a pull request