Remove built-in generator

This removes the `yo generator` generator in favor of [`generator-generator`](https://github.com/passy/generator-generator) as discussed in #761.

In order to merge this we should before:

* [ ] move `generator-generator` to the yeoman org
* [ ] update the [generators docs](http://yeoman.io/generators.html) and remove *old generator* references there
* [ ] update all docs to use `yo generator` instead of `yo generator:app` since there no longer is a name collision between the two

Re point one: Can I do this with my current permissions or should I first move it to someone else who can push it to the yeoman org in a second step? /cc @sindresorhus