test_2_6: _wrapcall() update and changed 2.7 test in test.py

- changed _wrapcall to do try-except instead of test on 2.7
- added dependency on unittest2 (for 2.6) in tox.ini (will PR soon) and import
  unittest2 in test.py for 2.6. Now test.py can use
  new unittest features transparently, unittest2 dependency not
  needed in setup.py for normal (not unittesting) users.