Register all the printers in one place (eg. in OpamSystem)
Improve the output of opam-mk-error on error.

This should help debugging #378
Do not use spawn a shell to do redirect when creating index.tar.gz. Apparently it can fail on some system and tar simply accepts an output file as argument.

This should fix #378
Merge branch 'master' of github.com:OCamlPro/opam