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.