Add leshan client

This is the first merge of our leshan client with the caveat that we need to resolve the last thing in leshan-client-standalone (which we will look at this evening).  But we wanted to get this out to you so you can take a look at the work :)

-The Zatarians (John, Simon, Josh, Andrew)
It's possible, but the node model in leshan-core is for data transmission, whereas the client node model maintains the object/object instance/resource model and their permitted operations.

What would we gain by setting up that extends relationship?