Make the 'Learn more' link customizable by creating a simple 'site.author_website' property.
'site.author_website' property is now optionnal: if user does not set it, 'Learn more' link won't be displayed anymore.
rename the site.author_website property into site.learn_more_link
Merge pull request #10 from Cylix/author_website_property

Added site.learn_more_link to _config.yml