Fix for #34

Fixes aria-hidden attributes so they're added when an item is closed, and removed when open.

Status: **Ready to merge**

Reviewers: @kpeatt @tedtate 

## Changes
- fixed adding aria-hidden attributes to bellows content wrapper
- added tests to ensure attributes where added and removed correctly
- bumped patch number in package.json

## How to test-drive this PR
- checkout this branch
- `grunt test`
- `grunt serve` and ensure attributes are added/removed correctly on close/open