When the program starts, it will go first main.m, and main.m function is mainly to do a bit of work:
1. Run the function main
2. Execute method UIApplicationMain
2.1 creates UIApplication object and set its agents
2.2 will open an event loop (main loop that is running an infinite loop, to ensure that the program does not exit)
2.3 loading infol.plist, which is determined there is no specified info.plist main, if specified, loading Main.stroyBoard;
View Main.storyboard among the arrow pointing the controller is displayed (if not specified, are not affected)
2.4 notify the application proxy, complete the application starts, execution didFinishLaunchingWithOptions