Add real functions to handle observe request.
Build system: move client and server in tests directory.
Build system: create an utils directory for commandline.c.
Test: add a command line interface to the client.
Test: use lwm2m_step() in the server.
Test: better usage of timeval for lwm2m_step().
New API: lwm2m_resource_value_changed() for a client to inform liblwm2m that one of its resource was updated.