Add sync operation for facefiles between /var/lib/AccountsService and ~/.face

This works around the problem with user-accounts in that we can't write a .face
file to another (not current) user's home directory.  We'll sync them up during
login when we have adequate permissions.
Merge pull request #36 from mtwebster/faces

Add sync operation for facefiles between /var/lib/AccountsService and ~/.face