minor mode hook and new grails-projectile-on

Hey there, 

Thanks for putting this on MELPA!

I added a minor mode hook that can be customized.  For me, this allows the loading of some grails yasnippets only when grails-projectile-mode is loaded, but I'm sure there could be other uses for this. :smile: 

I drew heavily from [projectile-rails](https://github.com/asok/projectile-rails/blob/master/projectile-rails.el) and added a `grails-projectile-root` that returns the root string if in a grails application.

This lets you establish the minor mode with 

```lisp
(add-hook 'projectile-mode-hook 'grails-projectile-on)
```

I also borrowed the code to disable grails in helm mini buffers.

You can still use global mode, but now this just turns on when you go to the project and projectile looks at it.

Great work, and again thanks for this. :+1: :+1: