Implemented OSGi support for leshan server

Bosch Software Innovations implemented OSGi support for the leshan lwm2m Server.

The leshan-osgi module provides an OSGi-fied implementation of the ClientRegistry called OsgiBasedClientRegistry, which can be used inside an OSGi container as a OSGi service. The LWM2M clients are registered with this registry and can be controlled over it. The implementation of ClientRegistryListener sent a OSGi Event via EventAdmin if registered / updated / unregistered method is called.
There are also some integration tests based on Pax Exam test framework for testing the basic functionality.
systemPackage(...) will not be necessary after merging the pull request #44