tox: add `cover` tox config

This configuration adds command-line test coverage reporting.
tox: add doctest flags to test configs

This adds missing --doctest-modules flags to a few test configurations
to ensure that we always collect doctests from the zpmlib source code.
tests: move tests/ package into zpmlib package
test_commands: fix a pep8 line spacing issue
.coveragerc: add cov config to ignore zpmlib.tests

The code in the zpmlib.tests package shouldn't be taken into account in
the measure of code coverage.
tox: fixed duplicate collection of tests

The `--doctest-modules zpmlib` directives were false, and causing tests
to be collected and run twice.
Merge pull request #55 from larsbutler/test-cleanup

Test cleanup