Add new slides with all overloadable operators

Added new slides in lecture 5  with further information on which operators can be
predefined.

Also corrected the outdated list with instance methods of the Enumerable class (now they are 48; +lazy). (again in lecture 5)
Merge branch 'add-slides-05' of https://github.com/yasen23/ruby-lectures into add-slides-05

Conflicts:
	lectures/05-classes-objects-enumerable.slim