Update webL10n to latest version (september 2013) (+support for attributes in mozL10n.get)

I needed a way to programatically get the translation for a tooltip, so I submitted a PR to webL10n: https://github.com/fabi1cazenave/webL10n/pull/38.

To avoid any regressions in getting the updated webL10n in PDF.js, I tracked down the base version of webL10n ((https://github.com/fabi1cazenave/webL10n/commits/0c06867a75) and looked at the diff: http://pastebin.mozilla.org/3061694. With this knowledge, I copied the latest version of webL10n (with the updates) to PDF.js and applied the changes.

I've checked each occurrence of mozL10n and webL10n, tested the results in the browser and didn't find any regressions, so it's safe to merge.