Run jekyll builds in parallel for each language

This should reduce `grunt jekyll:<appengine|devsite>` (all langs) to about 15 sec.

Pending issues to solve before merging:

* [x] `en` generation is fine. only `data-current-lesson` attr values are different but consistent 
* [x] generation for languages other than `en`
* [x] check sitemap.xml
* [x] don't copy over `_code` and `_assets` dirs: `master` branch doesn't
* [x] test `grunt develop`, i.e. `local.yml` config