Allow overriding of code highlighting language

At the moment, all of our code blocks default to being highlighted as
if they were Python.  This is because we add the 'python' class to
them all, which highlightjs picks up and uses to perform the
highlighting.  This patch allows one to select a different language
for an individual code block by labelling it with two classes:

 1) "override-lang" to prevent it from being labelled with the
    "python" class

 2) The class appropriate for the language one wants to highlight.
    For example, "cpp" -- which is why I'm adding this feature.
    (I want to be able to include some C++ from the default Ruggeduino
    firmware)

In markdown, one has to write the <pre> and <code> tags out manually
to achieve this.  A line of "~~~~" thus expands to this:

<pre><code class="override-lang cpp">

To get some C++ highlighted.

Change-Id: I08fa928c7205201bf921b422b73e01099777f927
Add section about custom robot object initialisation

Pyenv now supports broken up Robot initialisation. This section of the
docs gives a brief overview of how to break up the Robot
initialisation and also suggests why you may wish to do so.

Change-Id: I4217965fa8bd1ede1ad6b1ba12ff51b60514a7d9
Add Ruggeduino firmware to resources

Add the (slightly modified) Ruggeduino firmware for the custom firmware
documentation to the resources/kit directory.

The modifications from the stock firmware are:

* Defining FW_VER in the file so that it will compile
* Replacing "SRduino" with "SRcustom" in the firmware version string.

Change-Id: I7a21a59e561cbf31812a3cac0bbb50912c704587
ruggeduino-fw.ino: Correct comment about baud

There was a misleading comment that stated we communicate with the
ruggeduino at 9600 baud.

Change-Id: Ib44123b83043a005b97021b13252214cdfefb670
Document how to use a custom Ruggeduino firmware

Add an entry into the docs about how to use a custom firmware for the
Ruggeduino.  This explains the two avenues that one can go down:
extending the SR firmware, or using a completely custom firmware.

This commit is the combined work of Murray Colpman, Giorgos
Karatziolas, and myself.

Change-Id: I7b1cf7e78e6a610d2a0998f72f94f242dd1be0ef