Osso-backup support: instead of backing up the .modest folder, backup a tar.gz (fixes...

Osso-backup support: instead of backing up the .modest folder, backup a tar.gz (fixes NB#117612).

* src/hildon2/modest-prebackup.sh, src/hildon2/modest-restore.sh: these
  scripts create a tarball for the modest folder, to properly recreate
  the empty folders, and also exclude not needed files (local folder
  summaries).
* src/hildon2/modest.conf: now we include in backup the tarball file with
  .modest contents instead of the folder itself.
* configure.ac, src/hildon2/Makefile.am: install backup scripts properly.