Suggested fix for: ReferenceError: docpad not defined

This should potentially fix #4. Or more specifically in the generated js file if I add a `this.` in front of the reference to `docpad` on line 30 then it works. From that I assume that adding an `@` in the coffee script at line 18 should fix the problem.
Merge pull request #5 from pvandervelde/patch-1

Suggested fix for: #4 ReferenceError: docpad not defined