Update bottom text

Hello,

NOTE: This is the same as the pull request https://github.com/asciidoc/asciidoc/pull/7, but I cleaned up the commit and used a separate branch.

First things first, thanks for your great work on this tool.

As explained in both links I propose to change the behavior of the footer-text:
https://code.google.com/p/asciidoc/issues/detail?id=29
https://groups.google.com/forum/#!topic/asciidoc/EsOmfFrlGqQ

Context:
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I (and other people, for example: http://bugs.debian.org/656736) had the issue of the modification of the "Last updated" field at the bottom of the generated page that gives the modification date of the page (not the revision date) when you use asciidoc to generate HTML pages.

For example, I'm using a central git repo for the text pages and generate the html pages using a batch. When you update repo that is used for the batch it can be at another date than the date of the revision, so the "Last updated" field at the end of the page can be confusing.

My proposition:
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What I did in the patch is I used a new standard attribute named "footer-style" (you can change it if you want!) that could take 3 values:
  - default :or not defined, same as currently)
  - none : not displaying the footer-text section
  - revdate : displays the revision date instead of the source file modification date
Any other value will display only the version number from the footer-text section(if defined).

On Lex's advice I put all the logic in the asciidoc.conf and the backend files in order to only have a template call in the language files.
This time I updated all language files and documentation!

Please tell me if you have any remarks about this pull request.

Thanks in advance,
Joseph