Doing a little bit of searching through Christopher Hitchens’ own book God Is Not Great, he seems to be attributing this ditty to an English satirist. The reference is as follows:
Calvin’s Geneva was a prototypical totalitarian state, and Clavin himself a sadist and torturer and killer, who burned Servetus (one of the great thinkers and questioners of the day) while the man was still alive. The lesser wretchedness induced in Calvin’s followers, compelled to waste their lives worrying if they had been “elected” or not, is well caught in George Eliot’s Adam Bede, and in an old English plebian satire against the other sects ... who dare to claim that they are of the elect, and that they alone know the exact number of those who will be plucked from the burning:
We are the pure and chosen few, and all the rest are damned. There’s room enough in hell for you — we don't want heaven crammed.
As can be deduced from the tone, Hitchens himself is no objective source here, and he does not claim to be picking from a Christian source. Rather, he is quoting a satrical work, hyperbolically intended to ridicule the Pharisitical tendencies of their enemies.
As cited in JewishJournal.com, Hitchens attributes this ditty to “English Calvinists,” suggesting that this would come from the English Civil War, in the early to mid 1600s. During this period, Catholics and Calvinists were killing each other pretty fiercely, so the invective would not be out of place. In the same way that “damn yankees” would characterize their southern slave holding brothers as stupid, so too the Catholics would easily, for propagandistic reasons, have the desire to mischaracterize the doctrine of election as an elitist thing.
In truth, most who believe in Election do not believe that they know if they are elect or not (hence Hitchens’ explanation), and in no event do they believe that it is in any way a good thing that others go to hell. They view it as a sad fact, not a source of glee.
The Slate article perpetuates this stereotype, however, stating:
“As I pointed out a few weeks ago, Perry has not just accepted Jesus Christ as his personal savior, but has expressed the view that those who do not join him are headed for eternal damnation.”
The article is clearly written by someone who doesn't understand what Christians believe, for the idea that one would rejoice in the degenerate’s fate would only be held by the unregenerate in the first place.
Calvin could be a rather strict guy (I’m not sure I’d go so far as to characterize him as Hitchens did, but he could be rather stern), but these are not his own words. Rather, it was invective from the beginning, and the fair-minded Hitchens is merely repeating the drivel from 500 years ago.