feat(ios): add overrideUserInterfaceStyle to Picker

[m1ga]

The compact UI picker doesn't allow the user to set a text color. If you use your phone in light mode but have a dark background it will look like this:

This PR will allow you to switch the overrideUserInterfaceStyle for the Picker. I took most of the code/docs from the app overrideUserInterfaceStyle and modified it to use the picker instead of the whole app.

That way you can at least set the text color to white!

Test

var win = Ti.UI.createWindow({backgroundColor:"black"});

var picker = Ti.UI.createPicker({
  type:Ti.UI.DATE_PICKER_STYLE_COMPACT,
  minDate:new Date(2009,0,1),
  maxDate:new Date(2014,11,31),
  value:new Date(2014,3,12),
  backgroundColor:"transparent"
});
var btn = Ti.UI.createButton({
  title:"toggle", bottom: 40
});
var isLight = true;
btn.addEventListener("click", function(e){
  picker.overrideUserInterfaceStyle = isLight ? Titanium.UI.USER_INTERFACE_STYLE_DARK:Titanium.UI.USER_INTERFACE_STYLE_LIGHT;
  isLight = !isLight;
})
win.add([picker,btn]);
win.open();

• Run app in light mode
• click the toggle button to switch between the styles.