Release 0.5.1

With release 0.5.1, a wrong version number for gstreamer has been corrected (was 1.22, is now 1.20), and pom.xml files have been added to the repository modules to convince jitpack.io that these modules actually publish artifacts. It should now be possible to use jitpack.io to set java-gi modules as dependencies in an app's pom.xml or gradle.build without manually downloading the artifacts.