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`)
Actually you should not use `_stdErr` but leave `_stdOut`. This is not guaranteed to be a error, it can (and usually will be) the regular output of that command. 

There simply is no way to reliably fetch the error with `exec()`. That's why we've used `proc_open()` in the first place.