Moztrap 5931

This pull requests addresses issue #87.
Test case in Moztrap: https://moztrap.mozilla.org/manage/case/5931/

Also I did few other things in this PR:
* moved all imports from tests to module-level (PEP-8 compliance)
* made all tests destructive
* factor out part of test code into separate method for better re-use over several tests
* introduced new login option for returning users (interface to bidpom module)

I can later squash commits before merging it.