import: Add 'cran' importer.

* guix/import/cran.scm: New file.
* guix/scripts/import.scm: Add "cran" to 'importers'.
* guix/scripts/import/cran.scm: New file.
* tests/cran.scm: New file.
* Makefile.am (MODULES): Add 'guix/import/cran.scm' and
  'guix/scripts/import/cran.scm'.
  (SCM_TESTS): Add 'tests/cran.scm'.
* doc/guix.texi (Invoking guix import): Document it.
* po/guix/POTFILES.in: Add 'guix/scripts/import/cran.scm'.