Added accessor methods to chartLayout

The accessors means that we don't have to expose the structure
I've also added a class to the chartArea, having a 'g' selector felt a bit dangerous!

The layout is now used as follows:

```
var chartLayout = fc.utilities.chartLayout();

var setupArea = d3.select('#financial-chart')
    .call(chartLayout);

var plotArea = chartLayout.getPlotArea(setupArea)
```

If people are happy with this, I'll add documentation before merge. Let me know what you think @jleft