Extract the iOS modal into a separate module

[mmazzarolo]

Following #318 we should:

1. Extract the picker sheet "wrapper" modal in its own module (e.g. PickerSheetIOS).
2. Integrate PickerSheetIOS in react-native-modal-datetime-picker
3. Optional: Create a cross-platform PickerModalDialogWhatever that exposes the same APIs on both Android and iOS.

[mmazzarolo]

FYI @eightyfive this is still pending, want to release v8 first

[eightyfive]

Hey, thanks for the heads up.

Sheet wise, I took another direction since I wanted to be as close as possible to the native HTML <select /> feel when on Safari (iOS).

Hence I did put together this small package:

• index.ios.js
• picker-modal.ios.js
• modal.ios.js

[mmazzarolo]

Well, I still don't have enough time to work on it :/