Explicit size units

In this small PR I make `size` ignore unitless numbers as arguments. In other words `@include size(10);` will have no effect on the resultant CSS.

**Reason?** In CSS numeric widths and heights always have units. They're considered invalid otherwise… Being explicit about units here is good because it doesn't leave much space for imagination - for an instance I initially thought that `@include size(1)` gives a `1em × 1em` square. ;)

Hope you guys like it.