Added background for the SignColumn gutter

As I stated in a comment on your blog, I slightly modified CodeSchool.vim. I use the vim-gutter which uses a SignColumn gutter, and as this was not dealt with in the theme, it was displayed with a ugly light gray bg colour. I just added a line to fill that hole. Hope you don't mind :)

![codeschool-gitgutter](https://f.cloud.github.com/assets/769918/792765/80732fbe-ebc1-11e2-86d9-2335970329aa.png)