Twenty Twelve: Acronym tag should have underline/help cursor

[DrewAPicture]

As <acronym> tags serve a similar purpose to <abbr> and <dfn> tags and are structured the same, I propose we apply the same dotted underline and help cursor as the latter 2.

Patch attached.

[lancewillett]

Thanks – even though acronym is deprecated in HTML5 we might want to add support.

[iandstewart]

I'm cool with this. We've traditionally styled i as em and so on.

[lancewillett]

In [21445]:

Twenty Twelve: style acronym element same as dfn and abbr, props DrewAPicture. Fixes #21484.