Added p3 package to opam-repository

Can someone check that this is vaguely sensible? opam install p3 should provide a p3gen.native executable together with p3_lib.{cma,cmxa} libraries and other stuff in the lib dir. I had to learn a lot of technologies in the last few days (ocamlbuild, findlib, opam etc etc) so not really sure if everything will hand together.

Thanks!