Define @classmod wibox.widget.keyboardlayout, otherwise ldoc fails to
find the base class:
> module(...) name deduction failed: base .../src/build/docs/capi/ .../src/build/lib/awful/widget/keyboardlayout.lua
Fixes https://github.com/awesomeWM/awesome/pull/149#issuecomment-94014438
Widget shows current keyboard layout short name (usually two letters,
for example 'us', 'de'). Indicator will be hidden if only one layout
configured. Layout can be switched either by clicking on widget or by
function keyboardlayout.next_layout()