Add fixtures to marketplace-tests

Automatically log in to developer hub as needed
Split developer hub tests into two files
Create and automatically delete test apps as needed
Update requirements.txt to point to newest version of API client
Xfail test_that_a_screenshot_can_be_added due to newly uncovered bug
Make sure that test_packaged_app_submission cleans up after itself as well
Merge pull request #450 from bobsilverberg/use_api_1

Add fixtures to marketplace-tests