Load pages with universal newlines (mode 'rU')

If you manually add a document to the content directory which has \r\n endings throughout and then go to /index you will raise an IndexError on the line that says:

```self.body = self.content.split('\n\n')[1]```

Loading the content with universal newlines fixes that problem.

I'm working on other improvements to the handling of externally generated content, but that is a battle for another day.