How can I make a button have a rounded border in Swift?

I’m building an app using swift in the latest version of Xcode 6, and would like to know how I can modify my button so that it can have a rounded border that I could adjust myself if needed. Once that’s done, how can I change the color of the border itself without adding a background to it? In other words I want a slightly rounded button with no background, only a 1pt border of a certain color.