計算NSAttributedString的size

如果用先new好的textview來放，就會一直失敗

internal func calculateTextSize(attributedText: NSAttributedString, maxWidth: CGFloat) -> CGSize {

        var measureTextView = UITextView()

        measureTextView.attributedText = attributedText

        let size = measureTextView.sizeThatFits(CGSizeMake(maxWidth, CGFloat(FLT_MAX)))

        return size

    }