stbt.py: Check reply from lircd after sending key press

After sending a key press to a LIRC socket, wait for the reply packet
and check whether the command was executed succesfully. Raise an
exception unless lircd reports success.

`lircd` indicates the success of a command by sending the string "SUCCESS"
in its reply.

See: http://www.lirc.org/html/technical.html#applications