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 don't know if it would be relevant in this case, but I think we should try to use a common model as much as possible.

In that case, maybe the hierarchical relationship between the objects/object instances/resources could be shared (some work on the core model is probably needed).