tox: use py.test as test runner

Default unittest's testrunner is poor and shows useless tracebacks in
case something wrong with import statements or syntax.

In other hand, py.test is modern testrunner with cool output. It's used
often in open source python projects.

Resolves: #24
Merge pull request #25 from ikalnitsky/tox-pytest

tox: use py.test as test runner