.coveragerc: ignore test coverage for zpmlib/tests

The test code in zpmlib/tests should not be included in the collection
of test coverage metrics.
tox: revise test coverage configuration

This change removes the `cover` configuration and adds command-line test
coverage reporting for the py27 and py3 configurations. Without this
revision, test coverage is only run with python2.7.
tox: remove duplicated test collection

Having `zvshlib` specified in two places as an argument of `py.test`
results in duplicate test collection. This change fixes that.
tox: add `full` configuration to default env list
Merge pull request #25 from larsbutler/test-cleanup

Test cleanup
tox: put flags on single lines

This makes it easier to add new flags in the future.
tox: generate XML coverage report