Running `buildozer unknown_command` should show a meaningful error and
exit. Fixes regression introduced in 4936d31 and adds unit tests.
Also updates other tests `assert` keyword rather than `self.assert*` in
order to keep style consistent.
Last, minor `Dockerfile` documentation update as per recent @tshirtman
feedback.
- run setup.py on both Python2 and Python3
- runs simple test case for both Python2 and Python3
- checks the PEP8 style
- updates README.md adds Travis badge
