Initial po4a setup for translatable markdown documentation

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.