Swift4 api addition

[Nordeast]

This is a branch off of the swift 4 updated pull request. It adds a return value to all of the animate methods. The return value is the DaisyChain instance. This allows for a simpler syntax if the user prefers.

i.e.

let chain = DaisyChain()

chain.animate(withDuration: 0.5, animations: {
    view.center = CGPoint(x: 0.0, y: 0.0)
}).animate(withDuration: 0.5, animations: {
    view.center = CGPoint(x: 100.0, y: 0.0)
}).animate(withDuration: 0.5, animations: {
    view.center = CGPoint(x: 100.0, y: 100.0)
}).animate(withDuration: 0.5, animations: {
    view.center = CGPoint(x: 0.0, y: 100.0)
}).animate(withDuration: 0.5, animations: {
    view.center = CGPoint(x: 0.0, y: 0.0)
})