Udisks2 v2

This is new version of https://github.com/NixOS/nixos/pull/131, which adds udisks2 to the system. To summarize the knowledge:
* udisks2 will be used by gvfs after applying  https://github.com/NixOS/nixpkgs/pull/441. That should allow Thunar to mount/unmount removable media
* KDE has impure dependencies on udsisk1 so it should use existing udisk1 service and wait for better times
* It is possible to use gvfs with udisks2 disabled. gvfs just will not be able to see any removable media