Move OkupyError to okupy/__init__
switch back to non-relative imports, they proved to be confusing
Add flake8 support in runtests
Fix flake8 errors
okupy.tests is not an app anymore, and it's not needed by the discover runner
Add *.egg to gitignore
Enable ./setup.py test functionality
switch to ./setup.py test in travis
Run flake8 after successful test run in travis

Since it will run after successful test run, it will not block test run or
report any failure
exclude okupy/tests/__init__ from coverage
minor fix