Let oembed work directly as a submodule

For instance, this is what I run from `ckeditor-dev`:

    git submodule add https://github.com/shutej/CKEditor-oEmbed-Plugin plugins/oembed

I can then change the build configuration of `ckeditor-dev` to pick up the oembed module.

Having the plugin.js not in the root directory (as it is before this change) means that it's difficult to put it in the standard location.