Switch to setuptools

I think this is the recommended standard now, no?
Add tox development dependency

Install this with `pip install -e '.[develop]'`.
Update .gitignore
Add tests for plowshare module
Add tox config
Use explicit relative import for hosts

Implicit relative imports were removed from Python 3.
See http://legacy.python.org/dev/peps/pep-0328/.
Remove Python 2.6 testing

Rationale: subprocess.check_output was added in 2.7. If we want to keep
2.6 compatibility some workaround code will need to be added.