Add required sections to pom.xml for deployment

We will release this beast via Sonatype OSS... therefore, we need to
change the pom.xml a little bit. For details, see

	https://docs.sonatype.org/display/Repository/Central+Sync+Requirements
	https://docs.sonatype.org/display/Repository/How+To+Generate+PGP+Signatures+With+Maven
	https://maven.apache.org/guides/mini/guide-central-repository-upload.html

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>