Support poi control and screen latitude/longitude Delta

[42997455]

1. Add 'showMapPoi' to control the poi show on map.
   `<BaiduMapView
   ...
   showMapPoi={false}
   ...

`

2. We want to know the screen latitudeDelta and longitudeDelta for scope of map while mapStatusChanged.

[lovebing]

缺失 BMKPointAnnotationPro 实现

[42997455]

Thanks @lovebing merged the pull request, I find I just commit part of code for ios icon, so I create a new pull request #340 to commit the missing part code for the ios icon, could you please help me merge the missing part code for ios icon? Thanks!

Also thanks @summer-wu, the ios icon logic is from https://github.com/summer-wu/react-native-baidu-map .