unittest,tox.ini: catch and fix issue with trusty-level mock of open
unittest:
- Mock out open calls via builtin
- Create unittest/helper.py with mock_open context manager
- Handle module name difference between python2.7 and 3.0 via try-except
- Split wipe_zero and wipe_random into separate tests
tox.ini:
- add a 'trusty-py3' and 'trusty-py2' tox environment with more
specific versions as to what is on trusty.