Grouped Queue

Follow up PR to issue #433

I still need to:

- Run the prototype methods as part of their relevant queue group
- Fix issue where Grouped-Queue will always set default as the last queue (I'll change that in the plugin)
- Add a "run only once" option to Grouped-Queue so install is not called for every composed Generator

ATM, I add `#installDependencies()` in the end queue inside the `#run()` method. I wonder if this should not be taken care of inside `#installDependencies()` itself? (That would prevent issues where npm and bower are called multiple times if old Generators still call `#installDependencies()` manually).