Adding the flatten parameter to the PDF filling

It's there to enable the generated PDFs to be either flattened or
not. The reason for that is so that we can still run the same test suite
against the production.
Updated the README with chromedrive install on OS X
Hitting '/download?flatten=false' for testing

Using the argument to prevent flattening of PDFs
Merge pull request #228 from ministryofjustice/flatten

Flatten - optional