Add Gruntfile to bower’s ignore list.

Stencil’s Gruntfile is used only for development. This PR adds it to the list of Bower’s ignore files so it isn’t included in the installed package when running `bower install`. Fixes #37 .

Status: **Ready to merge**

Reviewers: @jeffkamo @kpeatt

## Changes
- Adds Gruntfile.js to Bower’s ignore list, so it won’t be included in the installed package.