Use po4a to provide translatable documentation

This commit add a new "docs-l10n" make target which uses po4a to:
* create .pot (PO templates) from markdown doc
* update templates and po for enabled languages
* generate translated markdown for completed (> 80%) translations

Currently, no language has been activated.

Signed-off-by: Luca Bruno <lucab@debian.org>
Generate initial translatable templates for documentation

These files are automatically genereated by `make docs-l10n` (via po4a),
which will also take of updating them if the original .md changes.

Signed-off-by: Luca Bruno <lucab@debian.org>
auto merge of #7641 : lucab/rust/lucab/po4a, r=catamorphism

As per https://mail.mozilla.org/pipermail/rust-dev/2013-July/004685.html 

This is the initial machinery to setup the l10n infrastructure for markdown documentation.
A new "docs-l10n" target will take care of generating, updating and then building .pot and .po files, and later on the final .md.

This commit includes the .pot for all current .md docs; they can be feed directly to Mozilla Verbatim if wanted.
Please note that po4a only provides the orig.md -> .pot -> l10n.po -> l10n.md flow. The l10n.md -> l10n.html generation is not currently built in the makefile, as no language has been enabled.