Add mw.html to Scribunto

A module for building complex HTML from Lua using a
fluent interface. The module is originally from enwiki,
but the authors allowed us to reuse it under GPLv2+
(as stated in the file).
The module will be loaded per default and comes with
unit tests.

As discussed on wikitech-l:
http://lists.wikimedia.org/pipermail/wikitech-l/2013-December/073320.html

Change-Id: I7c8d4378091c13d5ace0dd1fcbb4e27163e8c896
Merge "Add mw.html to Scribunto"