overhaul zooming system for x-axis zooming only

[jywarren]

or something

Ticks should scale intelligently, like in http://bl.ocks.org/stepheneb/1182434

Something to do with https://github.com/mbostock/d3/wiki/Quantitative-Scales#linear-scales

and stuff:

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

// also this seems important:

  x.domain(d3.extent(data, function(d) { return d.date; }));
  y.domain(d3.extent(data, function(d) { return d.close; }));