ENH: Compute lower number of points by default

Each new point can be expensive to compute depending on model complexity; five
points is to see overall shape of the curve.
Improved the learning curve example.

Scaled y-axis to make it easier to compare the two methods.
Wrapped duplicate code into a utility function that users can copy, paste
and adapt in their own project.
Added more iterations to cross-validate the GaussianNB model to get a smoother
curve.
Fixed style errors detected by pep8.
Merge pull request #3083 from sdenton4/enh_learning_curves

[MRG+2] Improved learning curves