A11y Improvements

Trying again with a11y improvements. Added relevant aria properties to the progress bar, added live region, added code to update `title` when slides update

Some notes:  
* progressbar changes have no impact on visual display
* new live region adds a className of 'offScreen'. I recommend theme developers use the following CSS for the live region

```
.offscreen {
  position: absolute !important;
  clip: rect(1px 1px 1px 1px);
  /* IE6, IE7 */
  clip: rect(1px, 1px, 1px, 1px);
}
```

These changes pass JSCS check.
Two unit tests are failing. I'm unsure whether they're related to the changes I've made, they do not appear related. If they are, let me know and I'll try to fix.