add stog 0.13.0
Merge branch 'master' of https://github.com/OCamlPro/opam-repository into stog
relax websocket dep for stog 0.13.0
stog: stog-server exe are optionally installed
stog < 0.13.0 require ocaml < 4.02.0
Merge branch 'stog'
Merge branch 'master' of https://github.com/OCamlPro/opam-repository
Merge pull request #2820 from zoggy/master

add stog 0.13.0