Handle symlinks

[DavideRossi]

Symlinks are ignored. They could handled à la git: in remote they become special text files containing the link path. When copied from remote they are restored as link in local. A problem with this approach is that we need a way to distinguish these special files. A possible solution is to use a specific additional extension (e.g. linkname.upback.symlink) but it is inelegant (and obviously it clashes with regular files that use the same extension).