Show section descriptions in section lists

- Update mustache to conditionally show descriptions
- Update section and subsection pages to show descriptions
- Update styles for descriptions
- Update navigate to use `currentTarget` rather than `target`.
  `currentTarget` will contain the element that jQuery is attaching the
  event to rather than the element within the link which was acutally
  clicked.
- Update `getTitle` to get the title out of a list item regardless of if
  it is in a heading in the list item or the list item is the title.
Merge pull request #21 from alphagov/section-descriptions

Show section descriptions in section lists