Fix indentation
Ignore .sass-cache directory
Remove and ignore memo/css/*.css from git
Use jekyll-sass

Without configurations for sass in _config.yml, jekyll-sass raises
an error.

I've copied and pasted in default configrations from README in
[noct/jekyll-sass · GitHub](https://github.com/noct/jekyll-sass).
Add a "Send a pullreq" link to _layout/post.html
Organize Gemfile