Performance plot for regression models

[mmastand]

Add a plot to regression models that plots actual value (x) vs. predicted value (y) and the line y=x.

User Experience

• Given I am a user that has a TrainedSupervisedModel of type regression
• When I call a method named something like .performance_plot() (or anything better)
• Then I should see a plot as described above.
• If I pass an optional save=True argument, then I should see a message saying that a file named regression_performance_plot_2017-12-10T05-33-53.png was saved. And of course the file should be saved. (png/jpg doesn't matter, just whatever is the default).

Notes

• Plots are hard to test, so test whatever you can that is reasonable (that a plot was saved?)
• Add documentation to the training document with an example plot.