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)
This way?:
https://gist.github.com/jvermillard/b8d7ca306bd49d10e203