Add Module to exports

I working on a `gulp` plugin for the current version of `es6-module-transpiler`.

Since `gulp` usually already reads the files, I am creating `Module` objects manually and adding them using `container.addModule` directly. However, the `Module` constructor is currently not exposed.

This PR simply exposes the `Module` for build tool development to not have to do: `require('es6-module-transpiler/lib/module')`

Please let me know, if you have something agains exposing the `Module` constructor.

Thanks for the great tool!