Added PagePublishLog

This pull request adds a ``PagePublishLog`` model to ``wagtailcore``.

This is helpful in finding out:
 - When a page was first published
 - When a page was last updated
 - Which revision is currently live

This builds upon my previous PR https://github.com/torchbox/wagtail/pull/566 so I don't have to implement this in three places. The commit that adds the publish log is this one: https://github.com/kaedroho/wagtail/commit/1b6f0b5e2652f89eed8db480f711f9a77f527a1d