Prepare for official publication

As discussed [on the mailing list](https://groups.google.com/forum/?hl=fr#!topic/scala-js/muO8MmKfUws), move packages around, and fill in organization and stuff.

I left the extensions in extensions, not mixin in anything into the package object of dom. This is the way it was in master. I leave Haoyi judge of adding an additional commit with that on top of this pull request if this is desirable.