important fix for sass port

whenever sass compilator sees statement like this: 
`margin: 0 -$variable;`

it will do math, before displaying variable's value, so finally we will get something like this (after source compilation ):

![uk-grid](https://cloud.githubusercontent.com/assets/1146430/4780052/916078f8-5c55-11e4-83d2-1232d9d1d3f1.png)

To prevent that behavior, some of the variables should be wrapped with parentheses: 
`margin: 0 (-$variable);`

This fix is very important, because sass-port contains broken grid system. Of course it doesn't affect less compilation.