Piped StdErr to StdOut when using exec and added method to digest errors

  - Now, when using exec, STDERR will be piped to STDOUT, giving the library access to errors. This way, the output of `proc_open` and `exec` is exactly the same.
  - To prevent differences in newlines between Windows and Nix, we use PHP_EOL
  - Added a protected method to parse and digest wkHtmlToPdf errors (for both `exec` and `proc_open`)
I thought it was only used by phpWkHtmlToPdf so, yeah, you're right. Error parsing is a must though.

Since one can extend Command class, one can override the getError method, for instance, and place parsing logic there. This would be useful in wkHtmlToPdf.

**example in Command class of phpWkHtmlToPdf:**

    public function getError()
    {
        if (!isset($this->_error)) return;
        
        $pattern = '/(?:Error\:)(?P<error>.*)/';
        $matches = array();
        if (preg_match($pattern, $this->_error, $matches) && isset($matches['error'])) {
            return trim($matches['error']);
        } else {
            return return $this->_error;
        }
    }



Another way is using some kind of injection method to provide a Command object with the Error Parser appropriate to the library. Using the Observer Pattern seems a good way. I will write something up in a gist to show you what I have in mind.