Adding class_names option to gain and lift plots

[MichaelFishmanOD]

I'd like to be able to set the names of the classes used in the legend in the the plot_lift_curve() and plot_cumulative_gain() functions.

I've made a pull request (#109) that adds an optional arg class_names to each of these functions to accomplish this.

This differs from issue #78 in that I am trying to change the labels in the plot's legend, not omit one of the classes from the plot.