tests: add support for tox

We also enable to run unittests from setup.py. This requires some
changes, like adding the needed dependencies but also we have to be
able to run setup.py with dependencies missing. We also need to ship
files needed by setup.py.
tests: make test work with Python 2.6 as well
tests: use Travis-CI for tests
Merge pull request #33 from vincentbernat/feature/tox

Use tox to run unittests