Support for Safari

I've been using TrelloScrum for a while on Chrome. Ever since I moved to the new MBP with Retina I had to move to Safari since Chrome wasn't rendering web content so well. One of the first browser plugins I started missing was TrelloScrum.

Since Chrome and Safari share Webkit I attempted to port TrelloScrum to Safari and came up with a very simple code change which makes this source now work with Safari as well.

I hope this change can help support TrelloScrum on Safari as well.