implements method for control over second y axis on Nvd3 charts (nCharts) of type "lineWithFocusChart"

 - implements yAxis2 method for Nvd3 objects to control styling
parameters (e.g. label, tickFormat) of second y axis
 - identical implementation to xAxis/xAxis2, i.e. yAxis2 get the same
parameters as yAxis by default
 - this is useful for changing the tick labels to percentage values
also on the second yAxis, or omitting them altogether, see code below
for these two example cases

data <- data.frame(x = 1:100, y = round(runif(100, 0, 1), 2))
n1 <- nPlot(y ~ x, data = data, type = 'lineWithFocusChart')
n1$yAxis(tickFormat = "#!  function(y) { return d3.format('01%')(y) }
!#")
n1 # same y2 axis tick labels as y1 axis

n1$y2Axis(tickFormat = "#!  function(y) { } !#")
n1 # no y2 axis tick labels