Deploy "api" and client modules to Maven Central

[nuest]

The core modules (the ones under ASL 2.0) and the client package should be deployed to Maven Central so that users can add them more easily (i.e. without adding 52N Maven repo).

Documentation: https://wiki.52north.org/bin/view/Documentation/HowToDeployArtifactsToMavenCentral

The example project should also be updated.