Test download links work using a proxy to inspect the network traffic 

This pull request is for code-review and feedback, so please do not merge it. It adds download link testing using BrowserMob proxy and network traffic. It depends on unreleased versions of pytest-mozwebqa and pytest-browsermob-proxy.

You can get these by running the following:

    pip install git+git://github.com/davehunt/pytest-mozwebqa.git
    pip install git+git://github.com/davehunt/pytest-browsermob-proxy.git