Display translator-credit info in the footer

xml2po has a feature to add translator credits to the articleinfo|bookinfo. This happens on translating XML source to a particular language, and we can process these tags. See example in #21 (closed).

But this xml2po's feature is limited to articleinfo and bookinfo, so I replace chapter tag artice. I think this satisfies us. All XML sources are valid Docbook again (hence title and subtitle is moved to articleinfo).

I have corrected many translator-credit infos to match expected regexp: "^([^<,]+)\s*(?:<([^>,]+)>)?,\s*(.*)$" → "NAME <EMAIL>, YEAR1, YEAR2". But there are some languages where translator-credit isn't set, and other where I can't correct the provided string, because the text should be understood (for example: Polish).

In the future maybe we can provide a mechanism to check tranlators-credits.

Footer layout: I experiment a middle aligned layout. I don't consider it a final solution. I plan to make final layout after resolving #24 (closed).

@ede123, please review it. Thanks!

Fixes #21 (closed)