add nvd3 x2Axis for lineWithFocusChart and tests

Addressing issue [#113](https://github.com/ramnathv/rCharts/issues/113), change nvd3 to accept x2Axis for type lineWithFocusChart.  Two files nvd3.R and chart.html both changed to provide the functionality.  Also, changed p7 in the nvd3 examples.r (test) to show how xAxis also changes x2Axis if type = "lineWithFocusChart" and x2Axis empty.