Swifty refactorings

[alwaysthecritic]

As I was studying this code I decided to refactor and improve a few things. In general I've tried to:

• make it Swiftier in style (so less of a straight Obj-C port)
• factor out repetitive code
• simplify a little, where possible
• encapsulate data/concerns within classes better, so each assumes less about the others.

Also, I've necessarily updated to latest Swift standards (2.1) so that XCode (7) was happy with it.