replaces queries on PHP_OS by stripos

stripos is case insensitive.
strstr in IncludePanel.php seems to make no sense since it doesn't use the return value of strstr
strpos in DebugTimerTest.php probably works in most cases but as noted on the Stack Overflow article http://stackoverflow.com/questions/738823/possible-values-for-php-os it can also return "Windows".

Fixes issue mentioned by Mark at https://github.com/cakephp/debug_kit/pull/78/files#r4691995