Don't say "obviously"

Closes #16.
Initialization vectors [come back] -> [appear]

Closes #15.
Don't pretend ECB flaws are obvious before explaining them

Closes #2.
Use emdash correctly

Closes #8.
Moved idealized encryption image to the end of the series to make the
progression more unidirectional

Closes #9.
Minor style matching

Closes #4.