Add min height and width

When omitting the height or width to have dc auto-size the charts, you have sizes that are smaller than the margins. This will result in negative lengths passed to d3 attribute calls, which in turn fail. We already have a default case when the sizes are 0, to use 200 instead. This patch extends that notion, such that any value less than the minimum is raised to the min and exposes the getter/setters.