Mark next release as 1.6.6.
Allow customization of display's width in Progress.
Fix warning about six unbundling.
Update version of `six` dependency.

In order to use the installed in Ubuntu 14.04.
Merge branch 'develop' of /media/med/merchise-git/xoutil into develop
Create an empty slot for next release.
Fix minor syntax (use local var for common values)
Improve `normalize_slug` function
Merge remote-tracking branch 'refs/remotes/merchise-usb/develop' into develop
Fix docstring indentation.
Add changelog about normalize_slug and normalize_ascii.
Release 1.6.6.