Simplify the isSubDirectory() function

isSubDirectory() checks if a specified $sub is a subdirectory of the
$parent, this is needed to prevent file inclusions.

Actually, the current code is more kind of a "hack" which I always
struggle over if browsing through source. So this should be a much
better implementation.

The implementation is really straightforward:
- [realpath()](http://php.net/manual/function.realpath.php) expands all
symbolic links and resolves references to '/./', '/../' and extra '/'
characters in the input path and return the canonicalized absolute
pathname.
- [strpos()](http://php.net/manual/function.strpos.php) returns FALSE if the
substring wasn't found.

**Since this is an absolutely critical piece of code, I'd like to ensure
that this is absolutely safe!** - Any comment appreciated!!!

@georgehrke I know that you implemented the existing method. What's 
your thought on this?