Post select consistency

The CMB_Post_Select internal get_posts method, uses get_posts( $query_args ), however the function used by the AJAX callback, uses new WP_Query( $query_args )

Both work OK, but the two take args in a different format, so (depending on your query args) you cannot switch between using AJAX and not without rewriting them.

I have updated the CMB_Post_Select internal get_posts method to use WP_Query instead. However I am wary of breaking things.

What do you reckon? @joehoyle