Add heading to model; small style changes

Hi there!

I added the heading to the announcement model, such that it can be customized per-announcement.  I also updated the style a small amount to account for bootstrap 3.  Finally, I relaxed the version constraint for Rails such that the gem can be used on Rails 4 again.

I didn't know if you wanted these updates, but I figured I'd share them anyway :)

Thanks for creating this gem, it's quite handy!