Le Soir update

Summary:
Le Soir now has three formats:

• the regular site
• the archives site archives.lesoir.be
• new archives on www.lesoir.be/archives

This change provides analysis code for the third, and refactors
the LeSoirPage and ArchivesLeSoirPage classes.

It also improves date handling.

Test Plan:
The three URL covers the three cases:

• http://archives.lesoir.be/alerte-a-la-penurie-estivale-de-medecins_t-20080730-00H5UN.html
• http://www.lesoir.be/1046282/article/actualite/regions/bruxelles/2015-11-17/un-sapin-21-metres-orne-grand-place-bruxelles-photos
• http://www.lesoir.be/archives?url=/regions/liege/2011-12-15/liege-la-dame-de-75-ans-est-decedee-883977.php

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D309