Log Message:
Updated package to version 2.8.4. Changes include:

 * Features/Changes
 ** Compiler: new jsoo_link binary to link multiple javascript files
    together and merging sourcemaps.
 ** Toplevel: improve genaration of toplevels, one can now pass a
    an export file containing units to export. (see jsoo_listunits)
 ** Lib: Provide a Js_of_ocaml (namespace) module to ease transition
    with future version of js_of_ocaml

 * Bug fixes
 ** Compiler: fix separate compilation when facing empty unit inside a library
 ** Runtime:  fix many javascript stubs
 ** Syntac: fix ppx deriving json