Activator download button

Hi Heather

Richard form Typesafe here. I was asked to fork scala-lang and add an Activator download button to the download page. Martin asked to have his name mentioned in the pull request, he'll be able to explain this request. 

I've added the download button. Because there are now two buttons on this page, I've changed the CSS to style the buttons via a class as opposed to an ID. 

kind regards

Richard

richard.summerhayes@typesafe.com