Refactor per WordPress coding standards

Replace cURL and allow_url_fopen() with wp_remote_get()
Add check to limit hours before now to 6 in shortcode
Add checks for empty/missing values
Better sanitizing

git-svn-id: https://plugins.svn.wordpress.org/aviation-weather-from-noaa/trunk@953892 b8457f37-d9ea-0310-8a92-e5e31aec5664