Expand collapse accessibiity

Making show/hide buttons more accessible:
![screen shot 2014-11-04 at 15 27 09](https://cloud.githubusercontent.com/assets/2579465/4902128/2c84c94a-6437-11e4-85c8-0d1b1f93f50a.png)

1. adding some text visible for screen readers: 'Hide section' instead of just 'Hide'
2. aria-expanded state
3. aria-controls linked to section id

+ unit tests
can go in an external text file, and used with `template` module