Scaffold site automatically has a vertical scrollbar

When you create a scaffold using `jekyll new PATH`, it'll generate a website like so:
![your_new_jekyll_site_and_main_css__testing](https://f.cloud.github.com/assets/4662860/1719927/dcaae9b2-61f4-11e3-8401-e9f3583d5204.jpg)

After [some](http://stackoverflow.com/questions/3557199/xhtml-html-element-with-100-height-causing-scrollbars) [googling](http://stackoverflow.com/questions/12989931/body-height-100-displaying-vertical-scrollbar), I find out that it's because of [margin collapse](http://www.w3.org/TR/CSS21/box.html#collapsing-margins). So, I removed the `margin-top` and replaced it with a `padding-top`.

Probably not a big issue but it was something I encountered and fixed for my Jekyll website.