Compatibility improvements

• re-compute menu position on device rotation
• custom options renderer
• menu children validation (required MenuOptions and MenuTrigger)
• popup menu is positioned above the menu trigger
• preserved order of menu children
• menu position computed using MenuContext dimensions instead of window dimensions
• option to enable debug logging

Initial public release

This version is initial public release of popup menu component that can be used as drop-in replacement of https://github.com/jaysoo/react-native-menu that should work with React Native 0.26 and additionally should not suffer of high CPU usage or ocational 'undefined' errors.