test_2_6: _wrapcall() update and 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 and import
  unittest2 in test.py for 2.6. test.py can use
  new unittest features transparently, unittest2 depency not
  needed in setup.py for normal (not unittesting) users
travis-unittest2: added unittest2, use mirrors

--use-mirrors is recommended http://about.travis-ci.org/docs/user/languages/python/#Pre-installed-packages

- added 2.6 and test in yaml
Merge remote-tracking branch 'origin/master' into test_2_6
Merge github.com:kislyuk/argcomplete
Merge github.com:kislyuk/argcomplete into test_2_6
test_2_6: - added test for FilesCompleter
- made completers:_wrapcall() python 2.6 compatible
- test.py changes for python 2.6

changed _wrapcall testing for 2.7 to try-except

changes per kislyuk
Merge branch 'master' of ssh://github.com/AvdN/argcomplete into test_2_6
Merge pull request #44 from AvdN/test_2_6

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