Support Jekyll 3.0

- Update to Jekyll v3.0
- Switch from Pygments.rb to Rouge for syntax highlighting
- Minor updates to syntax.scss partial and theme documentation
- Include jekyll-gist gem
- Close #192, #218

assets/css/main.scss

@@ -18,7 +18,7 @@ sitemap: false
 @import "normalize";
 @import "site";
 @import "typography";
-@import "pygments";
+@import "syntax";
 @import "coderay";
 @import "elements";
 @import "forms";