Install on dlpage

- As the copyright attribution for Typesafe has since been clarified, reverted the commit that removed it.
- Clarify on download page we're under standard bsd now.
- Moved installation instructions from getting started to download page.

To be honest, we'd like to get some more exposure for Activator.
Regardless, I think it makes sense to explain how to install right where the download link is,
so they don't have to leave this page to get started.

Happy to discus during this week's Scala meeting.

In the mean time, review by @heathermiller.