Fix some incoherences in the "griddle-build" mixin

Hi Nicolas,

When I reflected on [gh-12](https://github.com/necolas/griddle/issues/12 "Error in grid units"), I found some incoherences in the code. It works of course but these details are often disturbing for its readability and good understanding of code.

I hope you will accept these changes and good luck for your presentation at W3Conf.

Xavier.