Add 'guix archive'.

* guix/scripts/archive.scm, tests/guix-archive.sh: New files.
* Makefile.am (MODULES): Add 'archive.scm'.
  (SH_TESTS): Add 'guix-archive.sh'.
* doc/guix.texi (Invoking guix archive): New section.
* guix/scripts/build.scm: Export 'derivation-from-expression'.
* guix/scripts/package.scm: Export 'specification->package+output'.

Makefile.am

@@ -67,6 +67,7 @@ MODULES =					\
   guix/snix.scm					\
   guix/scripts/download.scm			\
   guix/scripts/build.scm			\
+  guix/scripts/archive.scm			\
   guix/scripts/import.scm			\
   guix/scripts/package.scm			\
   guix/scripts/gc.scm				\
@@ -130,6 +131,7 @@ SH_TESTS =					\
   tests/guix-gc.sh				\
   tests/guix-hash.sh				\
   tests/guix-package.sh				\
+  tests/guix-archive.sh				\
   tests/guix-authenticate.sh
 
 if BUILD_DAEMON