Paho changed the signature of on_connect to require 4 arguments as
of v1.3.0 (https://github.com/eclipse/paho.mqtt.python/issues/197)
Remove the "/#" from the subscription since it wouldn't match the
example topic which didn't include anything after rtl_433.
Adds a new interface for constructing, printing and releasing data_t*
structures. The data structure describes typed hierarchical data (much like
json) and then provides a function for dumping it to a file descriptor.
It comes with a small test.
