Coap shim

This is a first pass to a coap shim, the idea it to be able to offer a Bring your own coap client to leshan.  This is for the server only. The key elements are that there is a new module called connector.  This contains the Californium logic.  The leshan core contains the needed abstraction to build your own connector.  It also no longer import californium, only the connector does.

let me know what you guys think
This class is really thin, I think we could manage to remove this layer