Out-Of-Office supports ManageSieve backend

Two backends to store the Sieve script: file (current and default one) and ManageSieve. The latter requires sievelib to work.

This library is not packaged in common distros, but I've already packaged [1] for Ubuntu Trusty. There is no library in python for ManageSieve client. The default configuration for OOF does not require to have this library even installed.

It is now multi-tenancy ready by reading patched samba configuration key `auth:usernames are emails`.

Other fixes in OOF:
- Fix the SIEVE script when it was enabled/disabled.
- Keep user script as include if other active Sieve script was previously set.

[1] https://github.com/sixstone-qq/sievelib/tree/debian-pkg