Implemented post update date and reading time.

Update date is shown if it is defined in the post's front matter.
Merge pull request #73 from WrinklyNinja/post-metadata

Implemented post update date and reading time.
Add small class to minutes read meta data.
Ensure site title is centered on mobile, closes #75.
Grab form spree email from the site config. Closes #78.
Add related posts option, closes #41.
Add translate3d to transform, closes #74.
Fix whitespace.
Add OG tags, closes #55.