Fix cli https error

[jonblack]

This is related to #20.

The command line tool for mida doesn't correctly parse https urls, resulting in the failure to create an absolute url for images. This pr fixes the issue with the cli tool.

In issue #20 I had the same problem because I didn't pass a page url to Document.new, and no error is raised. Instead, Itemprop.make_absolute_url just returns an empty string.

That's by design, but it was certainly hard for me to track down, so I've updated the comment for Document.new.