Added a new method to the Pdf Class (pdf::buildCommand) so that we can g...

Added a new method to the Pdf Class (pdf::buildCommand) so that we can get the full exec command. Useful in windows environments so that you can bypass the proc_open PHP bug.
See this [Issue](https://github.com/mikehaertl/phpwkhtmltopdf/issues/56)

Also added a note to this in the documentation