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`
Set the codepage to 65001 before generating
Add check for running on Windows
chcp only needs to run once
Fix spelling error
Merge pull request #1443 from wickedshimmy/patch-1

Fix spelling error
Add note about current stable version of Octopress to README.

Fixes #1451.
Merge pull request #1431 from Haacked/windows-code-page-fix

Set the codepage to 65001 before generating
Merge branch 'master' of git://github.com/imathis/octopress into source