add a BundleActivator to start/stop a ManagedServer

This pull request proposes to add a BundleActivator to the californium-osgi bundle/project in order to start/stop a ManagedServer when the bundle is started/stopped. Requires the bundle http://central.maven.org/maven2/org/osgi/org.osgi.compendium/5.0.0/org.osgi.compendium-5.0.0.jar (and probably previous versions of it). The bundle has been tested with the Apache Felix Framework version 4.4.1 .