DONT MERGE: initial showcase implementation

This adds the showcase library and implements a showcase for the main activity and the log cache activity. If you try this locally, look at it the first time, as both showcases will only be displayed once. Remove "singleshot(...)" from the builder or change the showcase title to see it again.

Example: 
![image](https://cloud.githubusercontent.com/assets/406876/3926508/23faa4aa-23f2-11e4-83e8-2e88d7eed50e.png)

I's rather not change this explanation for that specific aspect. Instead, another highlighting of the specific button in the list activity will help more.