Added a compile warning and documented the limitation in the README.

Added Doxygen groups in CoAP and LWM2M.

Laurent Deru in #242.

output. Changed uip-debug to use the uiplib functions instead of its
own print functions.

number of bytes sent or error.

* Mark endpoint as secure regardless if DTLS peer exists or not
* Drop CoAP transmissions to endpoints that are not connected
* Debug output cleanup