minor mode hook and new grails-projectile-on

added a minor mode hook that can be customized.

Drew on examples from projectile-rails

 - added `grails-projectile-root` if the file is in a grails project root,
so that the mode can be loaded automatically when it enters into
projectile.

 - disabled `grails-projectile-mode` when in helm or other minibuffers.
Merge pull request #1 from jacobono/grails-projectile-on-and-hook

minor mode hook and new grails-projectile-on