Sphinx gettext

This branch supports translating documentation.

It adds compile-po and gettext target to sphinx/Makefile. gettext target makes pot files for documentation, and compile-po target compile translated po files.

To make translated documentation, run "make html" with LANGUAGE environment variable like this:
$ LANGUAGE=ja make html (for Japanese translation)

Japanese translation is written in the following site:
https://translations.launchpad.net/renpy-doc
Translated po files are here:
http://bazaar.launchpad.net/~vbkaisetsu/renpy-doc/renpy-doc-translation/files