Update sets-functions.tex

Fix in definition of set image.
Merge pull request #2 from m1kc/master

Different improvements
Defining implication in usual way

Also alternated formatting a little bit and defined environments \term
and \thm.
Знаки препинания, таблицы и рисунок пересечения множеств.
Merge pull request #3 from avsmal/master

Знаки препинания, таблицы и рисунок пересечения множеств.
Thrown away sets-union.png
Section 1.2 updated

Implication explained + layout fixes.
1.3 layout fix
Temporarily thrown out models and theories sections