new feature: links associated with apps.

I was looking for a way to link from Andlytics to the Google Play store so I could monitor things not tracked in the app -- like the "more by" and "also viewed" links, the G+1 counter, etc.

Then I realized that I look at a lot more than just the Play store: Amazon's web store stats, Flurry stats, Google+/Facebook pages dedicated to the app, and so on.

So, I added a new "links" feature for Andlytics. This feature allows you to add arbitrary URLs to list of links associated with an app. You can reach the links page by tapping the app's icon on the main screen.

This is my first contribution to Andlytics; please let me know if I should have handled this differently.