Keeps post original timestamp when regenerating all_posts/index page

[guites]

I noticed that if you clone your blog repository on a different machine, the first time you regenerate your blog (say when creating a new post) the original post dates on the index page/all_posts page will be lost.

I made a quickfix which orders posts by the timestamp that is written to the html files instead of their modified time (which changes every time you clone the repository).

Cheers