Call overridden version of `to_liquid` and remove duplicate code in `Octopress::Date`

Make sure to call "overidden" method which keeps us nice and compatible
so we don't need to keep up with whatever Jekyll usually does, and make
sure to include it in our overriden method.

Add method `liquid_date_attributes`.

Cleanup the Post and Page overrides to be more similar.

Normally I split these up into smaller commits, but the chages were so intertwined that it was nearly impossible todo so.