Rename package to js_of_ocaml

It's okay to keep the github repo name as is, but I think the `package.json` `"name"` field for precompiled binary packages has to match the non-precompiled version. This will make it possible to use shrinkwrap later, but even before then I believe it causes some issues.

For example, the `rebel` docs assume that the findlib name of a package is the same as the package.json name. 
