Small addition

I have added a factory interface for creating Connector objects which helps with decoupling of Californium from Scandium internals when setting up a DTLS based endpoint. I will re-factor the ManagedServer contained in californium-osgi to (optionally) use a ConnectorFactory for creating secure (i.e. DTLS based) connectors. A client application can then provide an implementation of ConnectorFactory for instantiating e.g. Scandium based connectors and wire it up with the ManagedServer.