Modular scale mixin and Music Intervals

I have added a font-size mixin that will allow us to use rem values with a pixel fallback for lesser browsers. 

I have also added  music intervals to both the modular scale function and the font-size mixin to help designers work with other analogies they are comfortable with. 

More on Music Intervals and Visual Design [here](http://24ways.org/2011/composing-the-new-canon)

I have only added to the modular scale function. What was there remains untouched and I tried to follow those conventions.