A bunch of changes

1 Remove all Sherlock library files and switch to Android official v7 library
2 Add Progressbar animation for refresh button in MainActivity
3 Add ShareActionProvider for StoryActivity so it's easy for user to share a story
4 Update the toolchain, including Android SDK Build-tools version and Gradle version. All changes should be compatible with latest Android Studio

Tested on Nexus 7 and Nexus 4 with 4.3