Reorganize project structure to enable Gradle-Builds

This is a major change so that the project follows the default [Android Gradle Structure][1]. This sadly introduces a problem with eclipse because eclipse can't recognize the correct source-directories. But all in all, this makes it easier to use Gradle within the project.

Also included is a travis-file, so if you have travis-ci enabled, it should create a travis-build automatically after the next push. Example: https://travis-ci.org/looperhacks/stolpersteine-android

[1]: http://tools.android.com/tech-docs/new-build-system/user-guide#TOC-Project-Structure