MKTextField error state support

[ryanfitz]

It would be great to be able to pass an error message to a text field to change the bottom border to a error state color and display the message below that.

textField.showErrorMessage("Username or Password is incorrect.")

To balance the api out, it would be nice to add a 'clearErrorMessage' function.

[gali8]

Look at https://github.com/gali8/G8MaterialKitTextField

[PhilippeBoisney]

Just a trick, but to handle error and show to user that textField is wrong I simply do that:

 txtFieldEmail.tintColor = UIColor.MKColor.Red
 txtFieldEmail.bottomBorderColor = UIColor.MKColor.Red
 txtFieldEmail.setNeedsLayout()