Added ability to see how long a user uses the app for to Analytics

Added in"time_in_app" property to the "Application Closed" event.
It would be nice IMHO if we put these lines in a method thus grouping them together and making the code more readable.

They would then be replaced by something with more meaningful for the reader, such as (for example):
```objective-c
[self trackApplicationClosed];
```

This is fairly minor but it would make modifications to `applicationDidEnterBackground:` easier.