Ensure we can set the min / max on time scales as well.
Improve the functionality of the margin accessor to allow you to set the margin properties in the following ways:

chart.margin({top:10,right:10,bottom:10, left:10});
chart.margin({left:30});
chart.marginLeft(30);
chart.margin(function(){
  return {top:10,right:10,bottom:10, left:10};
});
Merge branch 'master' of github.com:heavysixer/d4