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`)
We don't have to repeat the command in the error message. It's available from `getExecCommand()` or simply as `(string)$command`. Remember, this is only a base class.