Expose exported rending functions through the .renderer property of the given constructor.

So other rendering functions can call it like:

```
    var render = function (data, layer, options) {
        var renderer = this.column.renderer;
        var defaults = renderer.defaults;

        options = _.merge(options || {}, defaults, { column: { stacked: true } });

        return renderer.call(this, data, layer, options);
    };

    render.defaults = {
        stackedColumn: {
            stacked: true
        }
    };

    Contour.export('stackedColumn', render);
```

Fixed #193