Call overridden version of `to_liquid` and remove duplicate code

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.

I have had it with these monkey-patching methods in this Monday-to-
Friday language!

Add method `liquid_date_attributes`.

Cleanup the Post and Page overrides to be more similar.
Rename 'data' to 'date_attributes'

Prevents ambiguity, since there is already a `date` property defined in
the instance.
Merge pull request #1397 from IQAndreas/cleanup-date

Call overridden version of `to_liquid` and remove duplicate code in `Octopress::Date`
Merge remote-tracking branch 'octopress/master'
ignore source/_post which is output dir of Org-mode
update dependencies
Org-mode migration, Rakefile part
Org-mode migration: fix html links caused linebreak

Thanks to Org-mode's WYSIWYG on html links, some lines which contain
long html links are not that long.
Org-mode migration minor changes
Org-mode migration: fix code blocks