Reactive list

you may try this branch with the following pins
```
opam pin ocsigenserver git@github.com:ocsigen/ocsigenserver.git
opam pin tyxml git@github.com:ocsigen/tyxml.git#new_wrap
opam pin js_of_ocaml git@github.com:ocsigen/js_of_ocaml.git#new_wrap
```
plus a pin to an unreleased package
```
opam pin reactiveData https://github.com/hhugo/reactiveData.git
```