Add tests for checking that subpackages dont import each-other.

Note: there is an explicit import of vispy.app in vispy/gloo/__init__.py
to test that Travis picks up the test correctly.
Will remove that in a next commit...
Add test to travis.

Also add the import of vispy.app to gloo (forgot to add that)
Add comment to str.just command
Put import test under nosetest after all.

Just use a subprocess to ensure a clean sheet.