Extension for UIColor to initialize with an hexadecimal string written in Swift
usage:
... = UIColor.init(hexaStr: "696", alpha: 1.0)
... = UIColor.init(hexaStr: "#696", alpha: 1.0)
... = UIColor.init(hexaStr: "#669966", alpha: 1.0)
... = UIColor.init(hexaStr: "#669966", alpha: 1.0)