Style word count to match prototype
- Uses new style of naming things in CSS
- Also avoids edge case of duplicate IDs if there is more than one word counter
  on a page
Change the ARIA live property to 'polite'

`assertive` may result in the word count talking over the reading back of the text being typed.
Merge pull request #45 from alphagov/style-word-count

Style word count to match prototype