Layout tweaks, folder cleanup, updated screenshot

* Up to now there was no difference when using the app on phone or tablet. As the app doesn't show a lot of text, space on the tablet was not used efficiently. Some simple tweaks, mainly with paddings, might improve the user experience.
* The target sdk version of the app is 14. So the folders `values-v11` & `values-v14` weren't needed.
* The `readme.md` of this repository showed an old screenshot, not what the app looks now. Also the original one was cut off on the right side. Additionally cleaned the readme up, a bit.