Error parsing properties starting with operator (-/+)

Unfortunately using - operator in SASS causes this bug.

``` sass
$width: 10;

.test{
          margin-left: - 54 * $width - 1;
 }
```

Quick fixed with brackets:

``` sass
$width: 10;

.test{
          margin-left: - (54 * $width - 1);
 }
```
