Issue Description:

In Swift 2.2, we had:

func CGGradientCreateWithColors(
    _ space: CGColorSpace?,
    _ colors: CFArray?,
    _ locations: UnsafePointer<CGFloat>)
    -> CGGradient?

In Swift 3 this has changed to:

CGGradient.init?(
    colorsSpace: CGColorSpace,
    colors: CFArray,
    locations: UnsafePointer<CGFloat>)

The documentation for the locations parameter states that locations may be either:

1. NULL

2. An array of CGFloats with the same number of elements as the colors array

This new Swift 3 initializer does not accept `nil` for locations, which it probably should. In which case taking a type of `UnsafePointer<CGFloat>?` would be better. However, wouldn't an `UnsafeBufferPointer<CGFloat>?` be preferred? Although ideally it would accept `[CGFloat]?` and avoid unsafe pointers altogether.