Replace print statements with logging using mozlog

@whimboo 
This is to fix #26 
As pointed out in my last comment on this PR and what I also haven't known, we don't have to pass in the exception details into the exception() call. Which is great, so that we can define our own message, like `'Failure downloading add-on from %s' % url`.

Something I'm curious about is how this code behaves if the add-on cannot be found. If we require the add-on for our tests we should not only print the exception but probably exit the whole testrun. But that's clearly something for a new issue. Nothing you should cover here.