Implement chrome.system.storage for Android and iOS

- iOS: implement getInfo(), ejectDevice(), getAvailableCapacity()
- iOS: implement no-op support for onAttached/onDetached events
- Android: implement ejectDevice(), getAvailableCapacity()
- Android: implement onAttached/onDetached events for SD cards
- Update documentation to capture limitations of plugin
might be worth a comment here saying that case-sensitivity depends on file system.
http://stackoverflow.com/questions/6502712/isnt-android-file-exists-case-sensitive