Add support for OS X

[lukaskubanek]

#if os(OSX)
    import AppKit
    private typealias NSImage = LIImage
    private typealias NSSize = LISize
#elseif os(iOS)
    import UIKit
    private typealias UIImage = LIImage
    private typealias CGSize = LISize
#endif

[lukaskubanek]

Implemented in 8066b35