Add xScale parameter to Graph

By creating Rickshaw.Graph({xScale: d3.time.scale()}), users can now
instantiate a Graph with a linear scale that has Date objects as
domain values. This way, a regular Axis.X can render time using D3's
time formatter:

    var xAxis = new Rickshaw.Graph.Axis.X({
        graph: graph,
        tickFormat: graph.x.tickFormat()
    });