Allow ES6ification of specific packages.

I used the `container` package as a guinea pig because it was already
assumed to be AMD in the rest of the code, and it does not live
elsewhere as an ES6 project already.

This uses the `es6-module-transpiler` with a custom generator script
(found in `bin/transpile-packages.js`). The custom script does a few
things differently from the `compile-modules` script that comes with the
transpiler:

* Modify the generated package names so that it matches current build
  tooling.

  Examples:

  * A file named `packages_es6/some-package/lib/blahzorz.js` will create a module named
    `some-package/blahzorz`.
  * A file named `packages_es6/some-package/tests/blammo_test.js` will
    create a module named `some-package/tests/blammo_test`.
* Any modules generated from the packages `lib/` directory will be
  transpiled from the `packages_es6/package-name/lib/` directory to the
  `packages/package-name/lib/main.js` file. This file will contain all
  of the modules from the package.
* Any modules generated from the packages `tests/` directory will be
  transpiled from `packages_es6/package-name/tests/` directory to the
  `packages/package-name/tests/package-name_test.js` file.
* At the end of the transpiled test file, all of the testing modules are
  required (to ensure the tests are run).
Merge pull request #4338 from rjackson/make-es6-possible

Allow ES6ification of specific packages.