add coverage tox matrix with coveralls support
explicit coverage run for integration with coveralls
fix coverage tox env
tests: improved coverage reports with tests and migrations excluded from coverage
tests: fixed no cover pragmas on compat
tests: more steamlined configuration

* use new features of tox for shorter and better config
* add tests against python3.4
* do not use django-coverage because it caused false reports
tests: usedevelop=True in each environment to fix tox issue reintroduced in last commit
tests: solve multiple tests discoery issues:

* userena.contrib.umessages were run twice on tox testing
* userena tests for django<1.6 were not discovered at all
Merge remote-tracking branch 'upstream/master'