New repo structure with samples

We wanted to make it easier to get started with the Android library. Therefore we included some sample projects that were previously hidden in separate repos. 

This meant some big changes in the repository organization:
- moved the main library project to sense-android-library/ subdirectory
- merged in sense-android-demo project and sense-android-phonegap project into samples/ subdirectory (keeping history!)
- included Doxygen docs instead of hiding it in gh-pages