Travis: install argparse for Python 2.6
Travis: Only test the master branch
Fix 'TarFile' object has no attribute '__exit__'

See e.g. here:

http://stackoverflow.com/questions/6086603/statement-with-and-tarfile
Fix GzipFile instance has no attribute '__exit__'
Travis: install unittest2 on Python 2.6
Use unittest2 if it is available
Refactor assert_raises
Use test_raises from utils
Fix the last GzipFile error
Travis: remove "allowed failure" from Python 2.6
Merge pull request #50 from hashdist/py26

[WIP] Make hashdist work for Python 2.6