Fix issue with gutters for not first and not last columns

Test css: 

```scss
.content {  
  @include grid-span(12, 1);

  @include from(3) {
    @include layout(3 9) {
      @include grid-span(1, 2);
    }
  }

  @include from(4) {
    @include layout(3 8 1) {
      @include grid-span(2, 2);
    }
  }

  .thumbnails {
    $grids: 1;
    
    @for $i from 2 through total-slices() {
      $grids: add-grid($i at bp($i));
    }

    .photo {
      text-align: center;

      margin-bottom: 1em;

      @for $slice from 2 through total-slices() {
        @include at($slice) {
          @for $column from 1 through $slice {
            &:nth-child(#{$slice}n+#{$column}) {
              @include grid-span(1, $column, $output_style: float);
            }
          }
        }
      }
    }

  }

}
```

Result before fix (items starting from 2nd one displayed without margin-right):
![before](https://f.cloud.github.com/assets/108694/1394789/f32cad24-3c2e-11e3-9337-7f67f1921e92.png)

After:

![after](https://f.cloud.github.com/assets/108694/1394794/0204d128-3c2f-11e3-902f-4dc2729f02ed.png)