Passing custom styles to Picker

• Fixed style prop passing to pickers (PR #17 by @xxsnakerxx)

Picker value types and DatePicker component control

• Android Picker component now uses Integer and String as possible value (String added).
• Update DatePicker state when it receiving new props (PR #16 by @xxsnakerxx)
• Added order prop to DatePicker with ability to order date pickers (PR #16 by @xxsnakerxx)

Fix an extra year bug

• Fixed a bug with adding extra year when you providing maxYear prop (PR #14 by @hedaktibo)

Register custom datepicker for iOS

Now it provides registerCustomDatePickerIOS method to register custom datepicker component for iOS

Check if something changing

• Added componentWillReceiveProps to Picker component. (PR #12 @royipressburger)

Respect device density

• textSize and itemSpace props are using device density now (PR #10 by @rvgroup)

ViewPropTypes everywhere!

• Use ViewPropTypes with fallback in date picker too (not only in WheelCurvedPicker) (PR #7 by @Avid21)
• Fix PickerItem falsy labels (0, false, null, etc) and back to .toString() method only when label is undefined (PR #7 by @Avid21)

Fix View propTypes

• View.propTypes is deprecated since React Native 0.44.

Thanks @falsecz for PR and @mikaello for backwards compatibility fix.

Optimise styles and some refactoring

• Pass textSize, textColor and itemSpace properties down to datepicker
• Optimise some picker styles and remove styles that not used
• Remove unused iOS itemStyle property from Android WheelCurvedPicker
• Update demo application a bit

Fix nullable values

• Fixed a bug with falsy picker values (PR #3 by @Avid21)