Xattrs

this is the basic support for extended attributes in snapper. Maybe I should
give it more time before requesting to pull it upstream, but maybe (most propably:))
you'll notice some mistake or something I have missed during the developement
cycle. Feel free to ask me anything about the implementation.

(o) new status: XATTR
  - add 'x' at the end of status string

(o) adding 'xadiff' command into client (no dbus proto modifications)
  - the command reports modification in a very simple format
  - i.e: C:user.aaaa, D:user.bbbb, M:security.selinux for created, deleted or
    replaced extended attributes

(o) there's very simple testsuite for a few cases I had on my mind
  - basicaly, on non-btrfs filesystems, XAs have to be recreated during undochange
    command on status of TYPE,XATTRS or DELETED. for btrfs I think it's not neccesary,
    but I will test it later.

my short term TODO list on XAs:

  - think about checking XA support in filesystem during create-config and make
    this option immutable (undochange command from snapshot w/o XA support would
    ruin XAs otherwise)

  - create some performance report related to XAs

  - give btrfs and XAs more care