Use site.url instead of site.root

the css won't work if you use `site.root` when you use the project github pages(in the documentaions, you use user github pages), use `site.url` can fix this problem.

Please have a look and make some tests to see that whether it is fine or not. 

For example:
+ user github pages, local；
+ project github pages, local；
+ vps and more

you need to change the `url` in `_config.yml` according to different usages.