stbt.py: Check reply from lircd after sending a 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.

The structure of a lircd reply packet for a SEND_ONCE command is the
following:

BEGIN
<command>
(SUCCESS|ERROR)
[DATA
<error-code>
<error-message>]
END

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