Change screenshot basedir and support all screenshot formats, as well as screenshots in /assets

1) Change plugin basedir from s.wordpress.org/plugins to s-plugins.wordpress.org (the former seems to no longer exist).

2) Instead of assuming .png files located in the base directory, use fsockopen() to look for valid file URLs.

Following what the official plugin repo does, this first looks in /assets, then in the plugin's base directory, for any files with the extensions png, jpg, jpeg or gif.

If nothing was found, it leaves the description in the "Screenshots" section with a "missing image" note.