Various fixes.

fixes #430
Adding support for SHA256-CRYPT and SHA512-CRYPT.

fixes #450
fixes #493
Drop python 2.6 because unit tests refuse to start.

I need to find find the way to launch tests using python 2.6...
Prevent the creation of a domain alias using the same name as a domain.

fixes #498
Always set ownership.

fixes #499
Merge branch 'master' into feature/score-and-sorting-in-quarantine
A simple user can now see all its quarantined messages.

fixes #89