Remove all `<a>` in Workbook Markdown 

This cleans up the Markdown and ensures no HTML slips into content and
corrupts rendering. CSS attribute selectors now apply the same
Octicons, resulting in no visual change to the page.

Also, to simplify maintenance, a SASS mixin has been included for any link-Octicon styling:

```
@mixin octicon-link($char){
  font: normal normal 1em octicons;
  text-decoration: none;
  color: $color-dark;

  &:before{
    content: $char;
    margin-right: 5px;
  }
}
```

![screen shot 2014-04-23 at 5 34 48 pm](https://cloud.githubusercontent.com/assets/352082/2784580/e46ab7ea-cb3f-11e3-9edc-6fd83c67d565.png)