Compatibility with Js_of_ocaml 3.5 (ocsigen#622)

Compatibility with Js_of_ocaml 3.5

Merge pull request ocsigen#618 from ocsigen/dom-history-init

Dom history: do not cache the app splash screen

fix previous commit

release 6.9.1

impose upper version bounds on more js_of_ocaml-* dependencies

compatibility with OCaml 4.08.01

fix release

some fixes, convenience functions, and documentation

opam 2; upper version bounds

compatibility with tyxml.4.3.0