import wordpress' posts inclusive folder structure in the url

imports 2012/09/01/hello-world as 2012/09/01/hello-world,
not as 20120901hello-world

fixes #693
add changelog entry for #693
add myself to the contributors list
fix test_importing_posts_and_attachments for the new url structure
Merge pull request #1012 from evgeni/issue-693-wordpress-import-urls

import wordpress' posts inclusive folder structure in the url