make image urls absolute and added mock to test_requirements

I made a change so that when lassie.fetch is called with all_images=True the images src attributes contain absolute URLs. Since lassie already comes with a function that makes relative URLs absolute, I think it's better done inside lassie than in the application which imports it.

When trying to run the tests after the changes the mock package was missing, so I added it to the test_requirements.txt file.