Add Google+ enabled comments.

Based on:
http://dashburst.com/how-to-add-google-comments-to-any-webpage-or-blog-unofficially/
Merge pull request #268 from diwakergupta/gplus-comments

Add Google+ enabled comments.
Merge layout header data with page_data

This allows users to set default titles in templates.

I'm currently using this to get titles working in pagination.
Unfortunately, pagination pages don't have backing pages so there is no
better place to put the title (except in the config but that's even more
hacky)