add standard for unitless zero-value in CSS

I vote for introducing unitless zero-values.

1. it's superfluous
2. unit-less makes it easier to read, as it is more distinguishable from other values with a unit
3. the SASS compiler will strip the unit from 0 values anyway
4. it has become a widely used standard amongst developers
5. It's part of many styleguides, eg:
  1. [Github](https://github.com/styleguide/css)
  2. [Google](http://google-styleguide.googlecode.com/svn/trunk/htmlcssguide.xml#0_and_Units)
  3. [@necolas who created normalize.css](https://github.com/necolas/idiomatic-css#4-format)

AFAIK, TFG has always omitted units for zero-values.