How to update Exhibition

[tested01]

• [x] the latest posts

• [x] the most popular posts

[tested01]

The Latest Posts

• [x] Change ExhibitionView from component to container

• [x] Design corresponding action, reducer and state.

• [x] Implement update mechanism

• [x] Set the event trigger (candidate: Scrolling to the position above screen top)

• tips for future usage of scroll handler

1. 

2. Using react-native-timer to deal with async redux data update problem

[tested01]

TODO: Based on the latest post, do the same thing to the most popular posts

• [x] implement the API of fetching the most hot posts
• [x] refactoring: extract common method from update latest/popular posts
• [x] refactoring: extract method for the same type trigger mechanism