Modified X Axis to allow tickValues to be specified

Usage:
  var x_ticks = new Rickshaw.Graph.Axis.X.Values( {
    graph: graph,
    tickValues: // Array of x points at which to show a tick,
    orientation: 'bottom',
    element: document.getElementById('x_axis'),
    tickFormat: function(x){
      return x; // Formatter is called with each tickValue
    }
  });