change setupDimensions to convert 'margin' into an object

Hello, when setting the 'margin' attribute in a directive, like:
```html
          <nvd3-stacked-area-chart
            ...
            margin="50"
          </nvd3-stacked-area-chart>
```

I would see lots of errors in my console for:
Error: Invalid value for <svg> attribute width=“NaN”
Error: Invalid value for <svg> attribute height=“NaN”

I traced this to the width/height margin calculation when margin was set to a simple value. This patch will convert it into a proper margin object and fix those errors.