Implement caching

[GaryJones]

Creating an issue with a copy of code that will be cleaned up, since it would need to be tested and re-added. Pulled from the Zoninator class.

		// TODO: Caching needs to be testing properly before being implemented!
		function get_zone_cache_key( $zone, $args = array() ) {
			return '';

			$meta_key = $this->get_zone_meta_key( $zone );
			$hash     = md5( serialize( $args ) );
			return $meta_key . $hash;
		}

		function get_zone_posts_from_cache( $zone, $args = array() ) {
			return false; // TODO: implement

			$meta_key  = $this->get_zone_meta_key( $zone );
			$cache_key = $this->get_zone_cache_key( $zone, $args );
			if ( $posts = wp_cache_get( $cache_key, $meta_key ) ) {
				return $posts;
			}
			return false;
		}

		function add_zone_posts_to_cache( $posts, $zone, $args = array() ) {
			return; // TODO: implement

			$meta_key  = $this->get_zone_meta_key( $zone );
			$cache_key = $this->get_zone_cache_key( $zone, $args );
			wp_cache_set( $cache_key, $posts, $meta_key );
		}

		function _empty_zone_posts_cache( $meta_key ) {
			return; // TODO: implement
		}