Spike implementation of i18n html builds (ref: #139)


This is a spike implementation of i18n html builds (ref: #139).

Here is a summary:

* Rename backends/sphinx.py to backends/sphinx_html.py to import sphinx module from backends/sphinx.py.
* If `locale_dirs` is set at `conf.py`, collect .po files there and compile them with msgfmt, then run i18n html build with `-D language=<lang>` option.
* `sphinx-build`'s outdir is `_build/html/<lang>`.
* Documents are served from `rtd-builds/<version>/<lang>`.

I confirmed with https://github.com/papaeye/gntplib in a local environment (gunicorn), and the following URLs work well.

http://localhost:8000/docs/gntplib/en/latest/
http://localhost:8000/docs/gntplib/ja/latest/