add pull to refresh feature

resolve Refresh files list when pulling down #125
This PR uses [Android-PullToRefresh](https://github.com/chrisbanes/Android-PullToRefresh). In order to run the project in your IDE, please clone and add this as a library project to your project. After you cloned the repo [Android-PullToRefresh](https://github.com/chrisbanes/Android-PullToRefresh), you will find three project, only need to add **library**  to your project.