PHP optimizations for the shortcode in the plugin

Hi and thanks for a very nice working plugin 🙂
I have some optimizations for the plugin:

– I got a deprecation notice about Simple_Google_Photos_Grid_Shortcode::shortcode being called statically, it should be a public static function instead

– I also got a notice with a undefined index of non required attributes for the shortcode eg. “cache-interval”. We need to check if we have the key in the attributes before we’re falling back to the standard value for the attribute.

Else everything is working very fine 🙂