Fixed "unrecognized selector" error that would occur when using the text field in iOS 6.x.

When trying to use the text field in iOS 6.0 I got an error stating that an unrecognized selector (`tintColor`) has been sent to the text field. Now the text field checks wether `tintColor` is supported by the text field. The check I implemented is the same as the one in the text view; I just copied it.