Add lecture 04
Fix the overview slide of lecture 04

We were not able to get to the parallel assignment slides.
Format function result and remove some whitepaces in lecture 04.

Format function result in slide 36 for better view. Remove
whitespaces around operator ** in slides 40 and 45.
Add lecture 05
Increase the contrast in selected text

Selected text does not look okay on a projector. Override the background
and foreground colors of selections so that there is more contrast.
Add example for Array#fetch method with block in lecture 3
Use ←  instead of < for consistency
Add lecture 06
Add lecture 07
Add multi-value when example for case statement
Add multi-value when example for case statement
Add multi-value when example for case statement
Add multi-value when example for case statement