tox: use separate environments for pep8 and pyflakes

This should give us better errors when one but not the other fails.
Also named the environments to match the command executed.
tox: check setup.py too

When tox executes a command, it is not passed to a shell. This means
that glob patterns (e.g., *.py) wont work. We must thus explicitly
mention the Python files we want checked.
tox: specify pep8 and pyflakes deps explicitly

This avoids installing unnecessary dependencies into the virtualenvs
created for the pep8 and pyflakes tests.
tox: generate HTML coverage report

This will be published in Jenkins.
py.test: set --ignore once in setup.cfg
tox: generate JUnit XML report too

The "cover" tox environment was renamed to "full" to better reflect
that this is the test environment that will do full (extensive)
testing.
tests: add simple test
zpmlib: fix Python 3 compatibility

We didn't see these errors before because the source code wasn't
actually imported in Python 3 until we added the first test.
tox: find any Python 3 version on the system
Merge pull request #21 from mgeisler/tox

Tox enhancements