Adding some basic table styles

I needed some table styles for this post: https://dgmstuart.github.io/blog/2014/06/23/a-better-way-of-writing-commit-messages/

Not entirely sure I've done this correctly. I compiled the css with `compass compile --css-dir source/stylesheets --images-dir source/images -s compressed`, but in order to get it to run I had to temporarily comment out the `@import "plugins/**/*";` line in `screen.sass`