Extended matplotlib lesson. More information about managing figures and saving raster/vector graphics. Added more information about styling the plot (color, line and marker styles) and axes (scale, ticks, labels). Added a list of supported plot variants. Fixed some obvious mistakes.
Merge remote-tracking branch 'upstream/master' into extending-matplotlib-lesson

Conflicts:
	lessons/python/07-plotting-with-matplotlib.md
Fixed section levels to match changes introduced in the upstream.
Merge pull request #29 from turlog/extending-matplotlib-lesson

Extended matplotlib lesson