Adding "exclude" filter for themes; updating caching 

Fantastic plugin and really appreciate the code is so clean. These updates address to issues.  (1) the leading "http" or "//" was not enough for me excluding some scripts from minification.  (2) With a matching number of scripts (say 4) mis-matching cache files can get returned.  

If accepted the change allows themers to use something like below in mytheme/functions.php file to exclude scripts: 
function MYTHEME_asset_minify_exclude($queueHandle = '', $queueSource = 'http://') {
	$skipFile = false; 
	$skipFile = defined('WP_DEBUG') && WP_DEBUG === true ? true : $skipFile;
	$skipFile = strpos($queueSource, '.min') ? true : $skipFile;
	return $skipFile; 
}

We needed to exclude minify scripts on http://corbiscrave.com.  The second change was needed because our deployments left all scripts with the same maketime. If /search/ had 4 scrips and /recipes/ had 4 scripts, the original cache file was getting used. 

Again, thanks for the fantastic and clear classes.