Add generators directory to lookups

@necolas brought up a good point on irc about placing your generators in a generators folder  in your root together with a templates folder for instance. This would enable those who want to cleanly separate the generator code from the templates. There's already `lib/generators` in there now. Don't know if we should have both.

```
generators
  ˪ app
      ˪ index.js 
    controller
      ˪ index.js
templates
  ˪ index.html
    gruntfile.js
```

or

```
lib 
  ˪ generators
    ˪ app
        ˪ index.js 
      controller
        ˪ index.js
  ˪ templates
    ˪ index.html
      gruntfile.js
```