Version bump and use of Semantic Versioning

Use of Semantic Versioning as defined in http://semver.org.
Run ProGuard in optimize mode
ProGuard dump for v1.9.0-rc.1
Version bump and re-versioning

To prevent confusion with the old versioning scheme before the use of
Semantic Versioning (v1 is not at all related to v1.9.0), we now start
version at v5.0.0.

Since the app with versionCode 9 (v1.9.0-rc.1) had already been pushed
to Google Play, we cannot reuse the same versionCode. Therefore we are
forced to keep the previous versionCode and versionName, and restart
anew with versionCode 10 and versionName 5.0.0.