A Reason web toplevel built using js_of_ocaml

To make this word, you need

• The latest git version of BetterErrors: the one released on opam contains a typo preventing us to build correctly a javascript version of it.

• js_of_ocaml 2.7, haven't tested 2.8.

• Higlo must be installed, preferably from this branch to get reason syntax highlighting workinig: https://github.com/Engil/higlo/tree/reason-lexer