pytest: using pytest for testing (instead of unittest)

- rename test/test.py to test/test_argcomplete.py to match pytest default
  file pattern (test_*).
- arcomplete.py:
  * remove dependency on unittest and unittest2
  * replace TempDir with fixture tmpdir
  * use monkeypatch fixture so env. changes are not global
- update setup.py, so that 'python setup.py test' results in running py.test
- update Makefile to run py.test on target test and test3
- update tox.ini to include pypy, and reflect change of test.py name
- remove unittest2 from travis

for motiviation see my comment on https://github.com/kislyuk/argcomplete/pull/47#issuecomment-22825865