Add learning curves and plot_learning_curve

Add the function plot_learning_curve to tools and add the learning curve plot to 'Cross Validation' notebook. This plot provides insight into whether the number of training samples is large enough.

Fix #46

Conflicts:
	pymks/tools.py
Add learning curves and edit notebook `04 - Cross Validation`

Edit notebook `04 - Cross Validation`. Remove exercises from seminar
and provide examples for users to mimic. Add learning curves.

Adress #46
Merge branch 'issue46-learning-curves'

Fix #46

Conflicts:
	pymks/tools.py