Internationalizing Express docs

Proposal: Restructure docs to accomodate localization.

End result:

• Existing English URLs are unchanged.
• Localized content provided at http://expressjs.com/xx, where xx is the ISO lang code, e.g. http://expressjs.com/jp for Japanese.
• Other URLS would be e.g. http://expressjs.com/starter/installing.html --> http://expressjs.com/jp/starter/installing.html.

This would require some restructuring of the repository.

Current structure:

• 4x
• advanced
• api.html
• api.jade
• guide
• includes
• index.html
• index.jade
• resources
• starter

NOTE: The following content would not be translated (along with obvious stuff like package.json):

• 2x
• 3x
• images
• css
• js
• fonts