Refactor _config.yml
Use jekyll-sitemap gem

Delete the sitemap.xml and use jekyll-sitemap gem instead. Also I
start to use bundle to handle the dependencies here.
Refactor rakefile
Use constants to specify

which let us take the full advantages of RVM
New note
Migrate to SCSS
Customize SCSS
Add Github link
Prepare to refine layouts
Close #5
Shrink paddings