Make sure the width of the postView matches its superview.

Fixes #1001 

On the iPhone, when `[ReaderPostDetailViewController viewDidLoad]` runs in landscape orientation, `self.view.frame.size.width` has a value of 300, and this is what the `postView` starts with. The first time `[ReaderPostDetailViewController tableView: cellForRowAtIndexPath:]` runs in landscape orientation, `postCell.frame.size.width` is 320, giving us a deficit of 20px. 
Weird, eh?