RoundedButton.swift

import UIKit 

@IBDesignable
class RoundedButton: UIButton {

    @IBInspectable var cornerRadius: CGFloat = 0 {
        didSet {
            layer.cornerRadius = cornerRadius
            layer.masksToBounds = cornerRadius > 0
        }
    }
    
    @IBInspectable var borderWidth: CGFloat = 0 {
        didSet {
            layer.borderWidth = borderWidth
        }
    }
    
    @IBInspectable var borderColor: UIColor? {
        didSet {
            layer.borderColor = borderColor?.CGColor
        }
    }
}

Swift 3.0 update: .CGColor becomes cgColor