Enhancements on SegueProtocol

[marcelofabri]

The goal of this PR is making implementing prepareForSegue easier.

To do so, some changes were necessary:

• SegueProtocol inherits from Equatable
• UIStoryboardSegue implements SegueProtocol (identifier is now optional to match UIStoryboardSegue declaration)

Now, you can do something like this:

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue == ViewController.Segue.MySegue {

    }
}

[krzyzanowskim]

good job