UITextField text value observer experiments

gistfile1.txt

```swift
// SwiftNotes, observing via NotificationCenter
when(field, does: .UITextFieldTextDidChange) { [weak self] _ in
    log("UITextFieldTextDidChange (newValue: \(field.text)))")
    self?.textFieldChanged()
}

// MGObserver, observing via old school objc KVO methods
field.onChangeOf("text") { [weak self] in
    log("field.onChangeOf(text) (newValue: \(field.text)))")
    self?.textFieldChanged()
}

// Swift 4(?)'s new observer method? can't find it in the docs anywhere!
textObserver = field.observe(\.text) { [weak self] field, change in
    log("field.observe(text) (newValue: \(field.text)))")
    self?.textFieldChanged()
}
```