As you say, nobody is sure of who Jack the Ripper was, and therefore any quote attributed to him is unlikely to be real. The nearest thing we have to a quote from the Ripper are a number of letters sent to the police at the time. The majority of these are believed by Ripperologists to be hoaxes by journalists and troublemakers, but one letter is generally considered to be more believeable. This letter was sent on the 16th October 1888, at the height of the terror, and was addressed "From Hell", which is where the graphic novel and film get their name. Half a human kidney was sent with the letter and analysis at the time concluded it could have come from one of the Ripper victims, Catherine Eddows. The letter itself however, contains nothing similar to the quote at the beginning of the film and, as far as I am aware, the quote is purely an invention of the author/film makers. There are several Jack the Ripper websites on the net, one of the best being www.casebook.org.