Publish a 💎 theme

[ashawley]

I've investigated #307, but it would be some work. Here's what I have so far:

• Add pixyll.gemspec file
• Add gemspec to Gemfile
• Copy pixyll.scss to assets/css/main.scss
• Include favicons in assets/img
• Don't depend on github-pages, use smaller list of deps
• Create _layouts/home.html
• Tell users to create index.html instead of index.md
• Use layout: home in index.html
• Tell users to set paginate in _config.yml
• Move date format %b %-d, %Y from config to template.
• Drop conditionals in _includes/navigation.html Use site.header_pages for navigation #342
• Verify the rest of the includes are worth shipping?
• Ship the rake tasks somehow?
• Drop feed.xml in favor of jekyll-feed?
• Convert fb-instant-articles to a gem?
• Use gem in gh-pages rather than copying from master
• Convert pixyll.com to implementation of theme (same as previous?)
• Add .travis.yml in Add .travis.yml #355
• Build gem in Travis
• Test gem in Travis
• Add defaults for site.text values if not set in config

See WIP in #334