Update to 0.19

Update create commands for namespaces: vm, paas, ip, disk, vlan, webacc to
handle new datacenter status:

• prevent using a closed datacenter for creation
• display a warning when using a datacenter which will be closed in the future

Update 'gandi mailbox info' command: aliases are now sorted
Fixes #178: 'gandi account info' command now display prepaid amount
Fixes #185: 'gandi domain create' command can now change nameservers
Fixes #187: 'gandi record list' command has a --limit parameter
Fixes #188: broken links in README
Fixes certificate unittest for python3

Sponsored by: Gandi.net