Publish a 💎 theme

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

- [x] Add `pixyll.gemspec` file
- [x] Add `gemspec` to `Gemfile`
- [x] Copy `pixyll.scss` to `assets/css/main.scss`
- [x] Include favicons in `assets/img`
- [x] Don't depend on github-pages, use smaller list of deps
- [x] Create `_layouts/home.html`
- [x] Tell users to create `index.html` instead of `index.md`
- [x] Use `layout: home` in `index.html`
- [x] Tell users to set `paginate` in `_config.yml`
- [x] Move date format `%b %-d, %Y` from config to template.
- [x] Drop conditionals in `_includes/navigation.html` #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?)
- [x] Add `.travis.yml` in #355
- [x] Build gem in Travis
- [x] Test gem in Travis
- [x] Add defaults for `site.text` values if not set in config

See WIP in #334