[PDI-2343] Adding variables used in transformations and jobs

Improvements on the auto-documentation step :
  1 - cleaning the code to be in sync with Pentaho code style
  2 - changing the margin of the page and add comment as the page is LANDSCAPE
  3 - Add the generation date as page footer
  4 - Modify the ui to add a selector to display variables used by the trans / job
  5 - Modify the code to add the variables used by the trans / job inside the output