Cleanup: Remove unused variable from stbt.py
"irnetbox" python module to control the RedRat irNetBox-III infrared emitter

The irNetBox-III is a network-controlled infrared emitter:
http://www.redrat.co.uk/products/irnetbox.html

The infrared signal details come from a configuration file produced by
RedRat's "IR Signal Database Utility":
http://www.redrat.co.uk/software/SignalDBUtil/index.html

This doesn't support config files with "double signals" (where 2
different signals were recorded from alternate presses of the same
button on the remote control unit). For my use case, the IR Signal
Database Utility *did* record double signals, but my set-top box treats
them both the same, so I manually edited the generated config file and
replaced pairs of "DMOD_SIG" entries with a single "MOD_SIG" entry.

I haven't yet tested IRNetBox.irsend_raw with signals with a
"max_num_lengths" parameter (in the config file) other than "16".

Note that the irNetBox device only allows one TCP connection at a time,
so one client emitting IR signals could cause other clients to fail. So
we retry the TCP connection to the irNetBox 5 times with exponential
backoff.

Re. the `power` parameter of `irsend_raw`: Even at the maximum power of
100, my neighbouring set-top boxes do not see the signal sent to the
targetted set-top box. So even at 100 there won't be any IR interference
across different set-top boxes, unless your set-top box's infrared
receiver is more sensitive than mine.
stbt support for the RedRat irNetBox network-controlled infrared emitter

For details see the previous commit, and the change to README.rst in
this commit.

Note that the irNetBox device only allows one TCP connection at a time,
so `IRNetBoxRemote.press` holds the connection for as short a time as
possible. The sleep(0.5) is unfortunate, but my observations are that
sending a second signal before waiting 0.5 seconds results in the
irNetBox successfully sending the signal but the set-top box ignoring it
completely. Setting the "delay" parameter of the irNetBox
OUTPUT_IR_ASYNC message to 500ms and removing the sleep from
`IRNetBoxRemote.press` doesn't work: Multiple calls to `press` succeed
quickly one after the other, but the set-top box only understands the
very first one. The sleep of 0.5 seconds is still short enough that
multiple `press("2")` calls will be able to enter letters in a text box,
SMS-style.

Re. the exception handling in `IRNetBoxRemote._connect` see commit
5cd6ea4a.
Remove stderr output from irnetbox python module

To allow the caller to control what output is printed.

Note that `IRNetBox.__init__` still prints an error message when it
fails to connect and will retry after a short delay, to give the user
some feedback as to why it is taking so long. It is reasonable to see
this output from `stbt run` even without the `--verbose` flag.
Merge branch 'irnetbox'

Support for the RedRat irNetBox-III network-controlled infrared emitter:
http://www.redrat.co.uk/products/irnetbox.html

* irnetbox:
  Remove stderr output from irnetbox python module
  stbt support for the RedRat irNetBox network-controlled infrared emitter
  "irnetbox" python module to control the RedRat irNetBox-III infrared emitter