Added the rex_textSignal to UITextField

[RuiAAPeres]

No description provided.

[neilpa]

I hadn't considered this with the NSTextField version, but doesn't this create a circular reference that leaks the text field?

[RuiAAPeres]

I am testing it here, it seems fine, but on the other hand rex_pressed seems to be leaking:

[neilpa]

There's more to the retain cycle above than just rex_pressed since I've already got tests around that. I'm assuming that something with viewModel.authenticate is causing the problem.

[neilpa]

This looks good though, was worried about the notif center retaining the passed in object which it doesn't seem to be.

[RuiAAPeres]

Re-tested with only a UITextField:

        textField.rex_textSignal.startWithNext {
            print($0)
        }

And no leaks. 🐥

[RuiAAPeres]

Btw, I re-checked and the leak is coming from here:

        let username = NSUserDefaults.value(forKey: .Username)
        let password = NSUserDefaults.value(forKey: .Password)

If instead of this I just use regular strings, there are no leaks. 👍