A Swift UIColor extension for converting hex values to UIColors.
Converted from this StackOverflow answer (http://stackoverflow.com/questions/1560081/how-can-i-create-a-uicolor-from-a-hex-string/7180905#7180905).
let red = UIColor(hexString: "#f00")
let blue = UIColor(hexString: "0000FF")
let redWithAlpha = UIColor(hexString: "#F008")
let blueWithAlpha = UIColor(hexString: "#0000ff88")