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)
I think it would be great if we could use the same request/response model (from leshan-core) in both client and server. What do you think ?